Data Communication System, Computer-Implemented Method for Data Communication in a Railway Network, Computer Program and Non-Volatile Data Carrier TECHNICAL FIELD The present invention relates generally to drive arrangements for rail vehicles. Especially, the invention relates to a data communi- cation system for a railway network according to the preamble of claim 1 and a corresponding computer-implemented method. The invention also relates to a computer program and a non-volatile data carrier storing such a computer program. BACKGROUND When operating a rail vehicle, it is important to have accurate knowledge about the current adhesion conditions at the wheel- rail interface, i.e. the applicable kinetic friction coefficient. Name- ly, this is key for speed control in terms of retardation as well as acceleration. To improve the overall throughput and enhance the flexibility of railway traffic, it is envisaged that control systems will be implemented that employ so-called dynamic moving blocks to operate the rail vehicles in a railway network. In simplified terms, this means that the free distances between different rail vehicles are reduced substantially and will be set dynamically depending various parameters, such as the speeds and overall weights of the respective rail vehicles. Of course, the kinetic friction coeffi- cient on each rail segment is also an important parameter in this context. Provided that one has access to reliable values of the kinetic friction coefficient, the rail vehicles may be controlled to accelerate and decelerate in a highly efficient manner. Today, systems exist for informing rail vehicles about various characteristics of different track segments in a railway network, such that the rail vehicles may adapt their driving behavior accor- dingly. For example, US 2007/0219682 shows a system for providing at least one of train information and track characterization informa- tion for use in train performance, including a first element to de- termine a location of a train on a track segment and/or a time from a beginning of the trip. A track characterization element to provide track segment information, and a sensor for measuring an ope- rating condition of at least one of the locomotives in the train are also included. A database is provided for storing track segment information and/or the operating condition of at least one of the locomotives. A processor is also included to correlate information from the first element, the track characterization element, the sensor, and/or the database, so that the database may be used for creating a trip plan that optimizes train performance in accor- dance with one or more operational criteria for the train. WO 2022/006614 describes a method for improving braking per- formance of a rail vehicle, the method comprising the steps of: a) Associating one or more sensors with one or more components of the rail vehicle, at least one of the one or more sensors compri- sing an acoustic sensor configured to detect acoustic signals emitted from a wheel-rail interface; b) Acquiring measurements of one or more operating parameters of the rail vehicle using the one or more sensors; c) Transmitting the measurements of the one or more operating parameters to an operating parameter mo- nitoring device; d) Converting, using a computing circuit of the operating parameter monitoring device, the measurements into an output signal message including information relating to the one or more components of the rail vehicle and/or to adhesion condi- tions at the wheel-rail interface; and e) Transmitting the output signal message electronically to one or more recipients. US 2010/0023190 discloses a system for controlling a railroad train over a segment of track. The system comprises a first ele- ment for determining a location of the train on the segment of track; a second element for providing track characterization infor- mation for the segment of track; the track characterization infor- mation related to physical conditions of the segment of track; and a processor for controlling applied tractive forces and braking for- ces of the train responsive to the location of the train and the track characterization information to reduce at least one of wheel wear and/or track wear during operation of the train over the seg- ment of track. Although the above systems may offer dissemination of informa- tion with potential relevance for determining expected traction/ braking conditions, these system generally provide data of unsa- tisfactory quality. It is therefore not possible to keep the safety margins between the rail vehicles sufficiently short to obtain an optimal throughput of the railroad network. SUMMARY The object of the present invention is to mitigate the above prob- lems and offer a solution that enables rail vehicles to obtain up- to-date high-quality information about applicable adhesion condi- tions at the wheel-rail interface in various parts of a railway net- work. According to one aspect of the invention, the object is achieved by a data communication system for a railway network, which sys- tem contains at least one measurement controller, at least one transmitter apparatus and a set of receiver apparatuses. The at least one measurement controller is configured to be comprised in a respective one of at least one rail vehicle of a data-supplier type. The at least one measurement controller is configured to obtain a basic parameter reflecting an initial value of a friction coefficient relating to a rail segment, e.g. via an incoming messa- ge or as a default value. The at least one measurement controller is further configured to produce a validated value of the basic pa- rameter, which validated value reflects an updated friction coef- ficient between the rail of the rail segment and the wheels of the respective one of the at least one rail vehicle of the data-supplier type. The validated value, in turn is produced through a procedure involving: measuring individual rotational speeds of the axels to which the wheels of the at least one first rail vehicle are connected while applying a gradually increasing traction force to a specific one of said axles; determining, while applying the gradually in- creasing traction force, an absolute difference between the rota- tional speed of the specific one of said axles and an average rota- tional speed of said axles except the specific one of said axles, and in response to the absolute difference exceeding a threshold value deriving a parameter reflecting measured value of the fric- tion coefficient, checking whether the measured value of the fric- tion coefficient lies within an acceptance interval from the basic parameter, and if so assigning the validated value equal to the measured value of the friction coefficient. The at least one trans- mitter apparatus is configured to be comprised in a respective one of at least one rail vehicle of the data-supplier type and emit a friction data message containing the validated value of the friction coefficient. Each receiver apparatus in the set of receiver appara- tuses is configured to be comprised in a respective one of at least one rail vehicle in the railway network and receive the friction data message. The above data communication system is advantageous because it allows sharing of verified friction measures, i.e. information ba- sed upon which a rail vehicle carrying the receiver apparatus can rely when for example determining an appropriate distance to a rail vehicle in front. This, in turn, enables an improved overall throughput in the railway network. According to one embodiment of this aspect of the invention, each receiver apparatus in the set of receiver apparatuses is configured to receive the friction data message over a wireless interface, and each of the at least one transmitter apparatus is configured to emit the friction data message over the wireless interface. Thus, fric- tion data may be exchanged efficiently between rail vehicles, for example during travel in the railway network. According to another embodiment of this aspect of the invention, each of the at least one measurement controller is configured to produce a friction data message containing: the validated value of the friction coefficient, an identification of the rail segment to which the validated value of the friction coefficient relates, and a point in time when the validated value of the friction coefficient was derived. Thereby, any rail vehicle carrying the receiver appa- ratus may conveniently determine the relevant rail segment to which the friction coefficient relates as well as a reliability of the received information. According to yet another embodiment of this aspect of the inven- tion, each receiver apparatus in the set of receiver apparatuses is configured to receive at least two friction data messages relating to the rail segment, and derive an updated value of the friction coefficient for the rail segment based on the at least two received friction data messages. Here, the updated value of the friction co- efficient is based on a balancing of the validated values of the friction coefficient included in the at least two friction data messa- ges. For example, the balancing of the validated values of the friction coefficient may involve comparing the validated value of the fric- tion coefficient of each of the at least two received friction data messages to a threshold level for the friction coefficient, and as- signing the updated value of the friction coefficient equal to the validated value of the friction coefficient of a latest received mes- sage of the at least two received friction data messages, if the validated value of the friction coefficient of the latest received message is lower than or equal to the threshold level. If, however, the latest received message indicates a friction coefficient above the threshold level, the updated value of the friction coefficient is assigned a friction coefficient equal to the threshold level. As a result, it can be guaranteed that the friction coefficient is not as- signed an excessively high value. Alternatively, the balancing of the validated values of the friction coefficient may involve assigning the updated value of the friction coefficient equal to the validated value of the friction coefficient of a latest received message of the at least two received friction data messages without considering any threshold level. Thus, maxi- mum advantage can be taken of the validated value of the friction coefficient. According to still another embodiment of this aspect of the inven- tion, the system further contains at least one dispatchment node configured to receive the friction data message from one of the at least one transmitter apparatus via the wireless interface and re- lay the received friction data message to at least one of the recei- ver apparatuses in the set of receiver apparatuses via the wireless interface. A communication infrastructure in the form of such dis- patchment nodes is beneficial because it bridges distance gaps between different rail vehicles and thus ensures that the friction data message are distributed properly to the intended receiver ap- paratuses. Preferably, the at least one dispatchment node is configured to receive at least two friction data messages relating to the particu- lar rail segment, derive an updated value of the friction coefficient for the rail segment based on the at least two received friction data messages, and relay the updated value of the friction coeffi- cient for the rail segment to at least one of the receiver apparatu- ses. Analogous to the above, the updated value of the friction co- efficient is based on a balancing of the validated values of the friction coefficient contained in the at least two friction data mes- sages. Hence, friction data may be aggregated and enhanced in the at least one dispatchment node before being distributed to the rail vehicles in the railway network. In further analogy to the above, the balancing of the validated va- lues of the friction coefficient may involve assigning the updated value of the friction coefficient equal to the validated value of the friction coefficient of a latest received message of the at least two received friction data messages. Alternatively, the validated value of the friction coefficient of each of the at least two received fric- tion data messages may be compared to a threshold level for the friction coefficient, and only if the latest received message con- tains a validated value of the friction coefficient lower than or equal to the threshold level, the updated value of the friction coef- ficient is assigned equal to the validated value of the friction co- efficient of the latest received message. Otherwise, the updated value of the friction coefficient is assigned a friction coefficient equal to the threshold level. This advantageous for the same rea- sons as stated above. According to another aspect of the invention, the object is achie- ved by a computer-implemented method for data communication in a railway network, which method is implemented in at least one processing circuitry and involves: obtaining, in a measurement controller comprised in a rail vehicle of a data-supplier type, a basic parameter reflecting an initial value of a friction coefficient relating to a rail segment in the railway network, and producing, in the measurement controller, a validated value of the basic pa- rameter. The validated value reflects an updated friction coeffi- cient between the rail of the rail segment and the wheels of the respective one of the at least one rail vehicle of the data-supplier type. The validated value is produced through a procedure invol- ving: measuring individual rotational speeds of the axels to which the wheels of the at least one first rail vehicle are connected while applying a gradually increasing traction force to a specific one of said axles, determining, while applying the gradually increasing traction force, an absolute difference between the rotational speed of the specific one of said axles and an average rotational speed of said axles except the specific one of said axles, and in response to the absolute difference exceeding a threshold value deriving a parameter reflecting a measured value of the friction coefficient, checking whether the measured value of the friction coefficient lies within an acceptance interval from the basic parameter, and if so, assigning the validated value equal to the measured value of the friction coefficient. Moreover, a friction data message is emitted from a transmitter apparatus comprised in the rail vehicle of the data-supplier type, which friction data message contains the validated value of the friction coefficient. Further, the method involves receiving the friction data message in a receiver appa- ratus comprised in a rail vehicle in the railway network. The ad- vantages of this method, as well as the preferred embodiments thereof are apparent from the discussion above with reference to the proposed friction testing system. According to a further aspect of the invention, the object is achie- ved by a computer program loadable into a non-volatile data car- rier communicatively connected to a processing unit. The compu- ter program includes software for executing the above method when the program is run on the processing unit. According to another aspect of the invention, the object is achie- ved by a non-volatile data carrier containing the above computer program. Further advantages, beneficial features and applications of the present invention will be apparent from the following description and the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings. Figure 1 schematically illustrates a rail vehicle containing equipment that form part of a data communication system according to one embodiment of the inven- tion; Figure 2 shows a graph illustrating an example of the friction coefficient as a function of wheel slippage; Figure 3 shows a schematic railway network in which a data communication system according to one embodi- ment of the invention is implemented; Figure 4 shows a block diagram of a measurement controller according to one embodiment of the invention; and Figure 5 illustrates, by means of a flow diagram, a preferred embodiment of method according to the invention. DETAILED DESCRIPTION In Figure 1, we see a schematic illustration of a rail vehicle 100 containing equipment that form part of a data communication sys- tem according to one embodiment of the invention. Figure 3 shows a schematic railway network 300 in which the proposed data communication system may be implemented. The data communication system includes a set of receiver appa- ratuses. Figure 1 shows an example of such a receiver apparatus 110 that is comprised in the rail vehicle 100 and which receiver apparatus 110 is configured to receive a friction data message M(μ) from at least one other rail vehicle in the railway network 300. The friction data message M(μ) relates to a particular rail segment of the railway network 300, for example as illustrated by 310 in Figure 3. The data communication system also includes at least one measu- rement controller. Figure 1 shows an example of such a measure- ment controller 130 that is comprised in the rail vehicle 100. By definition thereby, the rail vehicle 100 is a rail vehicle of a data- supplier type, i.e. a source for producing validated friction informa- tion as will be described below. The measurement controller 130 is configured to obtain a basic parameter μ reflecting an initial value of a friction coefficient μ _e relating to the rail segment 310. For example, the basic parameter μ may be received in the receiver apparatus 110 via a friction data message M(μ) from another rail vehicle in the railway network 300. Then, the receiver apparatus 110 may forward the basic parame- ter μ to the measurement controller 130. Alternatively, or in addi- tion, the measurement controller 130 may produce the basic pa- rameter μ, for instance based on a default assumption, or an his- toric entry stored in the rail vehicle 100. Consequently, the basic parameter μ may originate from another rail vehicle in the railway network 300, dedicated friction test equipment performing mea- surements on the rail segment 310, or the rail vehicle 100 itself, for example through deduction based on neighboring measuring points. Nevertheless, the measurement controller 130 is configured to produce a validated value of the basic parameter μ. The validated value reflects an updated friction coefficient μe between the rail 140 of the rail segment 310 and the wheels 151, 152, 153 and 154 of the rail vehicle 100 of the data-supplier type. The validated va- lue is produced through a procedure involving the following steps. First, individual rotational speeds ω ₁ , ω ₂ , ω ₃ and ω ₄ are measured of the respective axels to which the wheels 151, 152, 153 and 154 respectively of the rail vehicle 100 are connected while applying a gradually increasing traction force AF to a specific one of the axles, say 152. Then, while applying the gradually increasing traction force AF to the specific one of the axles, an absolute difference is determined between the rotational speed ω ₂ of the specific one of the axles and an average rotational speed of the other axles, i.e. all the axles except the specific one of the axles. In response to the abso- lute difference exceeding a threshold value, a parameter μ _m is de- rived that reflects a measured value of the friction coefficient μ _e . The measurement controller 130 is further configured to check whether the measured value of the friction coefficient μ _e lies within an acceptance interval from the basic parameter μ, say ± 10 %, from the initial value of the friction coefficient μ _e . If the measured value of the friction coefficient μ _e lies within the acceptance inter- val, the measurement controller 130 is configured to assign the validated value equal to the measured value of the friction coeffi- cient μ _e . Of course, the acceptance interval need not be ± 10 %. On the contrary, any wider or narrower extension of this interval is likewise conceivable. If, however, the measured value of the friction coefficient μ _e does not lie within the acceptance interval, the measurement controller 130 is preferably configured to repeat the above steps to derive a new measured value of the friction coefficient μe. Referring now to Figure 2, we will explain how the validated value of the friction coefficient μe may be derived by studying the abso- lute difference between the rotational speed ω2 of the specific one of the axles and the average rotational speed of the other axles ω ₁ , ω ₃ and ω ₄ while applying the gradually increased traction force to the specific one of the axles. Figure 2 shows a graph illustrating an example of how the kinetic friction coefficient μ _k is expressed as a function of the wheel slippage s, which here is understood to designate a common term for a spinning or sliding motion of the wheel relative to the rail resulting from an applied traction or bra- king force respectively. In other words, the wheel slippage s is applicable to retardation as well as acceleration. Characteristically, the kinetic friction coefficient μ _k increases re- latively proportionally with increasing wheel slippage s. When ap- proaching a peak value μ _e , however, the kinetic friction coefficient μ _k levels out somewhat. The friction coefficient peak value μ _e is associated with an optimal wheel slippage s _e after which a further increase of wheel slippage s results in a gradually reduced kinetic friction coefficient μ _k . According to the invention, a parameter μ _m is determined that re- flects the friction coefficient between the rail vehicle’s 100 wheels and the rails upon which the rail vehicle 100 travels. Ideally, the peak value μ _e should be derived. For example, the peak value μ _e may be derived as follows. When the absolute difference between the first and second wheel speed signals ω ₂ and ω _a exceeds the threshold value, this corresponds to a situa- tion where the at least one wheel 152 on the specific one of the wheel axles experiences a wheel slippage s _m near the optimal wheel slippage s _e . The kinetic friction coefficient μ _k is given by the expression: where F is the force applied by a drive unit to accelerate the rail vehicle 100 m is the mass of the rail vehicle 100, and g is the standard acceleration due to gravity. Under the assumption that the wheel slippage s _m is near the opti- mal wheel slippage s _e , the peak value μ _e of the kinetic friction coefficient μ _k may be estimated relatively accurately. In addition to the above, the data communication system accor- ding to the invention includes at least one transmitter apparatus, which in Figure 1 is exemplified by the unit 120 comprised in the rail vehicle 100 of the data-supplier type. The transmitter appara- tus is configured to emit the friction data message M(μ _e ) contai- ning the validated value of the friction coefficient μ _e , such that at least one other rail vehicle in the railway network 300 may obtain information about said validated value by receiving the friction da- ta message M(μ _e ). Consequently, each of the at least one rail vehicle 100, 312, 313 and 314 in the railway network 300 that is equipped with a receiver apparatuses 110 can make use of the validated value of the fric- tion coefficient μe, for example when keeping a particular distance to a rail vehicle in front, accelerating and/or when braking. It is preferable if the measurement controller 130 is configured to produce the friction data message M(μ _e ) such that it contains not only the validated value of the friction coefficient μ _e , however also an identification of the rail segment to which the validated value of the friction coefficient μ _e relates and a timestamp designating a point in time when the validated value of the friction coefficient μ _e was derived. Figure 3 exemplifies this by showing a friction data message M(μ _e , ID ₃₁₀ , t ₁ ) including an identification ID ₃₁₀ of the rail segment 310 to which the validated value of the friction coefficient μ _e relates, and a point in time t ₁ when the validated value of the friction coefficient μ _e was derived. Preferably, each receiver apparatus 110 is configured to receive the validated value of the friction coefficient μe over a wireless interface via the friction data messages M(μe), and each transmit- ter apparatus 120 is configured to emit the validated value of the friction coefficient μ _e over the wireless interface via the friction data messages M(μ _e ). Namely, this allows for convenient sharing of high-quality friction information between the different rail ve- hicles 100, 312, 313 and 314 in the railway network 300. To maintain accurate and updated records about the adhesion conditions at the wheel-rail interface in the railway network 300, according to one embodiment of the invention, each receiver ap- paratus 110 is configured to receive at least two friction data mes- sages M(μ _e ) relating to the same rail segment, say 310, and derive an updated value of the friction coefficient for the rail segment 310 based on the at least two received friction data messages M(μ _e ). The updated value of the friction coefficient is based on a balan- cing of the validated values of the friction coefficient μ _e included in the at least two friction data messages M(μ _e ). Technically, how- ever, the balancing may involve any kind of weighing together of the validated values of the friction coefficient μ _e , such as calcula- ting average or median value. According to one embodiment of the invention, the balancing of the validated values of the friction coefficient μ _e involves com- paring the validated value of the friction coefficient μ _e of each of the at least two received friction data messages M(μ _e ) to a thres- hold level for the friction coefficient. If the validated value of the friction coefficient μ _e of a latest received message is lower than or equal to the threshold level, the updated value of the friction coef- ficient is assigned equal to the validated value of the friction coef- ficient μ _e of the latest received message of the at least two re- ceived friction data messages M(μ _e ). Otherwise, the updated va- lue of the friction coefficient is assigned a friction coefficient equal to the threshold level. Thus, the friction coefficient will never be assigned a better/higher value than the threshold level. This facili- tates complying with regulatory requirements that may prescribe maximum values for the friction coefficient. Alternatively, according to another embodiment of the invention, ^{the balancing of the validated values of the friction coefficient μ} _e involves assigning the updated value of the friction coefficient equal to the validated value of the friction coefficient μ _e of a latest received message of the at least two received friction data messa- ges M(μ _e ), i.e. without considering any maximum value for the fric- tion coefficient. Referring again to Figure 3, according to one embodiment of the invention, the data communication system contains at least one dispatchment node 320. Each dispatchment node 320 is configu- red to receive the friction data message M(μ _e , ID ₃₁₀ , t ₁ ) from at least one transmitter apparatus 120 via the wireless interface. Here, a rail vehicle 100 comprises a transmitter apparatus 120 that emits the friction data message M(μ _e , ID ₃₁₀ , t ₁ ). Moreover, each dispatchment node 320 is configured to relay the received friction data message M to at least one of the receiver apparatuses 110 in the set of receiver apparatuses via the wireless interface. Here, a respective receiver apparatus in each of the rail vehicles 312, 313 and 314 respectively receives the friction data message M(μ _e , ID ₃₁₀ , t ₁ ) from the dispatchment node 320. Consequently, it is sufficient for the rail vehicles 100, 312, 313 and 314 to be com- municatively connected to at least one dispatchment node 320 in order to exchange friction information with other rail vehicles in the railroad network 300. According to one embodiment of the invention, the dispatchment node 320 is configured to receive at least two friction data messa- ges M(μ _e , ID ₃₁₀ , t ₁ ) relating to a particular rail segment, say 310. The dispatchment node 320 is further configured to derive an up- dated value of the friction coefficient for the rail segment 310 ba- sed on the at least two received friction data messages M(μ _e , ID310, t1). Analogous to the above, the updated value of the friction coefficient is based on a balancing of the validated values of the friction coefficient μe comprised in the at least two friction data messages M(μe, ID310, t1). The dispatchment node 320 is configu- red to relay the updated value of the friction coefficient for the rail segment 310 to at least one of the receiver apparatuses 110 by emitting a friction data message M over the wireless interface. In further analogy to the above, according to one embodiment of the invention, the balancing of the validated values of the friction coefficient μ _e involves comparing the validated value of the friction coefficient μ _e of each of the at least two received friction data messages M(μ _e , ID ₃₁₀ , t ₁ ) to a threshold level for the friction coeffi- cient, assigning the updated value of the friction coefficient equal to the validated value of the friction coefficient μ _e of a latest recei- ved message of the at least two received friction data messages M(μ _e , ID ₃₁₀ , t ₁ ), if the validated value of the friction coefficient μ _e of the latest received message is lower than or equal to the thres- hold level. Otherwise, the dispatchment node 320 is configured to assign the updated value of the friction coefficient to a friction co- efficient equal to the threshold level. Alternatively, according to one embodiment of the invention, the balancing of the validated values of the friction coefficient μ _e ef- fected by the dispatchment node 320 simply involves assigning the updated value of the friction coefficient equal to the validated value of the friction coefficient μ _e of the latest received message of the at least two received friction data messages M(μ _e , ID ₃₁₀ , t ₁ ). Figure 4 shows a block diagram of the measurement controller 130 according to one embodiment of the invention. The measure- ment controller 130 is configured to receive the wheel speed sig- nals ω ₁ , ω ₂ , ω ₃ and ω ₄ , and ω _a and output a signal defining the traction force AF to be applied as well as a value of the updated friction coefficient μ _e . The measurement controller 130 includes processing circuitry in the form of at least one processor 430 and a memory unit 420, i.e. non-volatile data carrier, storing a com- puter program 425, which, in turn, contains software for making the at least one processor 430 execute the actions mentioned in this disclosure when the computer program 425 is run on the at least one processor 430. In order to sum up, and with reference to the flow diagram in Fi- gure 5, we will now describe the computer-implemented method for data communication in a railway network 300 that is carried out by the measurement controller 130 according to a preferred embodiment of the invention. In a first step 510, it is checked whether a basic parameter μ has been obtained, which basic parameter μ reflects an initial value of a friction coefficient μ _e relating to a particular rail segment 310 in the railway network 300. If the basic parameter μ has been ob- tained, for example via a received friction data message M(μ), steps 520 and 530 follow; and otherwise, the procedure loops back and stays in step 510. A validated value of the basic parameter μ is produced by execu- ting steps 520 to 570. The validated value reflects an updated friction coefficient μ _e between the rail 140 of the rail segment 310 and the wheels the rail vehicle 100 of the data-supplier type, i.e. in which the procedure is executed. In step 520, a rotational speed of a specific one of the wheel axles of the rail vehicle 100 of the data-supplier type is obtained, and in step 530, preferably parallel to step 520, an average rotational speed of the axles except the specific one of the axles is obtained. In a step 540 following step 520, a traction force AF is applied to the specific one of the axles, and in a step 550 subsequent to steps 540 and 530, it is checked if an absolute difference between the rotational speed of the specific one of the axles and the ave- rage rotational speed of the axles except the specific one of the axles exceeds a threshold value. If so, a step 560 follows; and otherwise, the procedure loops back to steps 520 and 530. Next time, when reaching step 540, the traction force AF is applied at a somewhat larger magnitude than last time, such that for each run through the loop the traction force AF is gradually increased. In step 560, a parameter μm is derived, which reflects a measured value of the friction coefficient μe. The measured value of the fric- tion coefficient μ _e is derived as described above referring to Figure 2. Thereafter, a step 570 checks if the measured value of the friction coefficient μe lies within an acceptance interval from the friction coefficient μ _e reflected by the basic parameter μ obtained in step 510. If the measured value of the friction coefficient μe is found to lie within the acceptance interval, the validated value of the basic parameter μ is assigned equal to the measured value of the fric- tion coefficient μ _e , and a step 580 follows. Otherwise, the proce- dure loops back to steps 520 and 530 for deriving a new measured value of the friction coefficient μe. In step 580, a message M(μ _e ) containing the validated value of the basic parameter μ is emitted, for example over a wireless in- terface, so that it may be received by other rail vehicles in the railway network 300. After that, the procedure loops back to step 510. All of the process steps, as well as any sub-sequence of steps, described with reference to Figure 5 may be controlled by means of a programmed processor. Moreover, although the embodi- ments of the invention described above with reference to the dra- wings comprise processor and processes performed in at least one processor, the invention thus also extends to computer programs, particularly computer programs on or in a carrier, ad- apted for putting the invention into practice. The program may be in the form of source code, object code, a code intermediate source and object code such as in partially compiled form, or in any other form suitable for use in the implementation of the pro- cess according to the invention. The program may either be a part of an operating system, or be a separate application. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a Flash memory, a ROM (Read Only Memory), for example a DVD (Digital Video/Versatile Disk), a CD (Compact Disc) or a semi- conductor ROM, an EPROM (Erasable Programmable Read-Only Memory), an EEPROM (Electrically Erasable Programmable Read-Only Memory), or a magnetic recording medium, for ex- ample a floppy disc or hard disc. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or by other means. When the program is embodied in a signal, which may be conveyed, directly by a cable or other device or means, the carrier may be constituted by such cable or device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the relevant proces- ses. The term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components. The term does not preclude the presence or addition of one or more additional elements, features, integers, steps or components or groups thereof. The indefinite article "a" or "an" does not exclude a plurality. In the claims, the word “or” is not to be interpreted as an exclusive or (sometimes referred to as “XOR”). On the contrary, expressions such as “A or B” covers all the cases “A and not B”, “B and not A” and “A and B”, unless otherwise indicated. The mere fact that certain measures are reci- ted in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limi- ting the scope. It is also to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed in- vention, from a study of the drawings, the disclosure, and the ap- pended claims. The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.