TITLE "A RAILWAY SLEEPER" FIELD OF THE INVENTION This invention relates to a railway sleeper formed from polymer concrete. In particular, the invention relates to a railway sleeper that may be used as a replacement for timber, steel or concrete sleepers in existing railway lines or as a sleeper in new railway lines. BACKGROUND OF THE INVENTION Worldwide, the railway industry installs millions of sleepers each year. A significant number of these sleepers are used to maintain existing lines while the rest are used to construct new lines. The materials traditionally utilised for new sleepers are timber, concrete or steel. Timber sleepers do perform well, but changes in environmental awareness and rapidly diminishing supplies of high quality hardwoods have reduced their long held advantages. Concrete sleepers have gained increasing acceptance, but their stiffness typically limits their use to locations where complete sleeper replacement is undertaken or where new track is constructed. Due to their increased depth (approximately 280mm compared to 120mm for timber) concrete sleepers have a much higher stiffness than timber sleepers, making them unsuitable to be interspersed with timber sleepers. Due to this increased depth, concrete sleepers also require significantly more ballast than timber sleepers. The high weight of concrete sleepers (about three to four times that of timber sleepers) makes them expensive to transport and difficult to handle with conventional sleeper replacement technology. Steel sleepers can be interspersed with timber sleepers but they are generally too light, have poor bearing characteristics, and require high maintenance. OBJECT OF THE INVENTION It is an object of the invention to overcome or alleviate one or more of the above disadvantages or provide the consumer with a useful or commercial choice. It is the preferred object of this invention to enable railway sleepers to be produced with a similar depth/size as hardwood timber sleepers. It is a further preferred object of the invention to allow railway sleepers to be produced with similar stiffness characteristics as hardwood timber sleepers. It is a still further preferred object of the invention to allow railway sleepers to be produced with a weight similar to that of hardwood timber sleepers. It is a still further preferred object of the invention to allow railway sleepers to be produced cost effectively. It is a still further preferred object of the invention to allow railway sleepers to be produced that have excellent durability and are able to resist biological and chemical attack. It is a still further preferred object of the invention to allow railway sleepers to be produced that have no environmental restrictions that affect storage, handling and eventually disposal. SUMMARY OF THE INVENTION In one form, although not necessarily the broadest or only form, the invention resides in a railway sleeper comprising: a body having a top on which the railway rails are located and a base for placement on a ground surface; the body being produced from a polymer concrete having an amount of polymer resin and an amount of filler; the body including at least two lower tension zones located adjacent the base of the body and at least one upper tension zone located adjacent the top of the body between the two lower tension zones; wherein the amount of resin in the polymer concrete is higher in the tension zones when compared to the amount of resin of the polymer concrete in the remainder of the body. Preferably, the lower tension zones are located substantially under where the railway rails are to be on the sleeper. Preferably, the concrete is polymer concrete (or filled resin system). The filler may include an amount of a light aggregate with a specific gravity less than that of the resin and an amount of a heavy aggregate with a specific gravity larger than that of the resin. The resin may be any suitable polyester, vinylester, epoxy or polyurethane resin or combination of resins dependent on the desired structural and corrosion resistant properties of the polymer concrete. Preferably, the resin content is between 25-30% by volume. The light aggregate with a specific gravity less than that of the resin can be any type of light aggregate or combination of light aggregates dependent on the desired structural and corrosion resistant properties of the polymer concrete. Usually, the light aggregates have a specific gravity of 0.5 to 0.9. The light aggregates usually make up 20-25% by volume of the polymer concrete. Preferably, the light aggregate are centre spheres. The centre spheres normally have a specific gravity of approximately 0.7. Alternately, hollow glass microspheres with a similar specific gravity and volume may be used. The heavy aggregate with a specific gravity larger than that of the resin can be any type of heavy aggregate or combination of heavy aggregates dependent on the desired structural and corrosion resistant properties of the polymer concrete. The heavy aggregates usually make up 40-60% by volume of the polymer concrete. Preferably, the heavy aggregate is basalt. Usually the basalt is crushed. The crushed basalt may have a particle size 1 to 7 mm. Preferably, the basalt makes up between 40-50% by volume of the polymer concrete. The basalt normally has a specific gravity of approximately 2.8. Alternately, natural or artificial sand that has a similar specific gravity as basalt may be used. Alternatively, the heavy aggregate may be made up of one or more of coloured stones, gravel, limestone, shells, glass or the like material. Preferably, the polymer concrete contains a thixotrope to keep the light aggregate in suspension. The polymer concrete of the present invention may also include fibrous reinforcement material to increase the structural properties of the polymer concrete mix. The reinforcement material may be glass, aramid, carbon, timber and/or thermo plastic fibres. Preferably, the railway sleepers include fibre composite reinforcement members located within the body. More preferably, the fibre composite reinforcement members extend through the tension zones. Preferably, the fibre composite reinforcement members are pultruded members. The pultruded members may be tubular or have a U- shape, L-shape or thin rectangular shape in transverse cross-section. The fibre composite members may have the majority of their fibres orientated in longitudinal direction. The fibre composite reinforcement members may be produced from any suitable glass, carbon or aramid fibre and/or plastic material dependant upon the desired properties of the railway sleeper. Preferably, the surface area of the fibre composite reinforcement members that contact the polymer concrete is abraded to increase adhesion between the polymer concrete and the fibre composite reinforcement members. Alternatively, the fibre composite reinforcement members may be coated with a sand and/or gravel interface to increase adhesion. Preferably, the fibre composite reinforcement members have flat surfaces to simplify the sanding or abrading process. In another form, the pultruded fibre composite reinforcement members may be filled with standard concrete, polymer concrete or a filled resin system with or without a metal or fibre composite reinforcing bar to further increase their load carrying capacity and stiffness. In another form again, the pultruded fibre composite reinforcement members may be filled with other materials dependant upon the desired properties of the reinforcing element. The pultruded, fibre composite reinforcement members may be filled before placement into the mould. One or more attachment members for attaching the railway rails to the sleeper may form part of the sleeper. The attachment members may be those currently used in the art such as any of the Pandrol's cast-in shoulders. In another form, the invention resides in a method of producing a railway sleeper formed from castable material, said method including the steps of: locating an amount of polymer concrete in a mould, the polymer concrete including a resin and a filler; wherein the polymer concrete is located at different locations so that the resin is able to migrate and create tension zones that contain a higher amount of resin when compared other areas of the sleeper. The method may further include the step of placing fibre composite reinforcement members within the mould. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention, by way of examples only, will be described with reference to the accompanying drawings in which: FIG. 1A shows a perspective view of a railway sleeper according to an embodiment of the invention; FIG. 1 B shows a top view of the railway sleeper of FIG. 1 A; FIG. 1 C shows a front view of the railway sleeper of FIG. 1 A; FIG. 1 D shows a side view of the railway sleeper of FIG. 1 A; FIGS. 2A to 2E shows front views of the steps required to produce a railway; FIG. 3A shows a schematic view of a sleeper without any loading; FIG. 3B shows a schematic view of a sleeper with loading; FIG. 3C shows a schematic view of a sleeper the tensile and compression zones that are created due to the loading shown in FIG. 3B; FIG. 3D shows a schematic view of a sleeper with fibre composite reinforcement members; FIGS. 4A to 8A shows a top views of sleepers using different fibre composite reinforcement members; and FIGS.4B to 8B show side view of sleepers using different fibre composite reinforcement members. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1A to 1 D shows a railway sleeper made of polymer concrete. The railway sleeper 10 includes a body 20 that has a top 30 that includes placement of a railway rail and a base 50 used to place the sleeper on a bed of gravel. The sleeper has a rail seat 40, two ends 60, a middle section 70 and a number of wings 80. The railway sleeper also has two lower tension zones 90 and a single upper tension zone 100 located between the lower tension zones. The lower tension zones 90 are located within the rail seat 40 adjacent the base 50 of the railway sleeper 10. The upper tension zone 100 is located adjacent the top of the railway sleeper 10. The railway sleepers 10 are produced within individual moulds which provides opportunity for shape optimisation. Polymer concrete is far more expensive than concrete and every litre of material saved results in a significant saving in dollars. Hence, the size of the cross-section of the rail seat 40 has been reduced compared to normal trapezoidal concrete sleepers. It is important that there is 180mm width at the top of the rail seat 40, and a minimum of 220mm width at the base of the rail seat 40. In order to comply with this requirement, the cross-sectional shape at the rail seat, shown in FIG 1 D, has been used. This shape saves several litres of polymer concrete compared with a traditional trapezoid shape and still has 180mm at the top of the rail seat 40 and 220mm at the base of the rail seat 40. The sleeper 10 also has a reduced height in the middle section 70 of the sleeper. Bearing area is only required under the railway rail. The middle section 70 is only required as a tie to hold the two rail seats 40 together. By ensuring that there is enough bearing area under the two railway lines, the width requirement of the middle section is determined by handling requirements. Most sleepers are installed mechanically and machines need to grab a sleeper without breakage. Furthermore, the middle section 70 needs to have enough strength and stiffness to hold the gauge of the railway line within a narrow tolerance. Hence, middle section 70 has a width of 125mm and a height to 80mm By reducing the height of the middle section 70 to 80mm compared to the 115mm of the rail seat 40, the middle section 70 is able to be covered by gravel. This provides the advantage that during a derailment the wheels of the train are less likely to contact and damage the sleeper. The wheels will run along the gravel on top of the middle section. The ends 60 of the sleeper are a sufficient distance away from the application of the load by the railway rail so that the forces towards the end are much smaller than closer to the railway rails. Hence ,the cross- sectional area at the ends 60 of the sleeper 10 have been reduced. The highest tensile stress is in the base of the sleeper under the railway rails, i.e., the base of the rail seat. Hence, the wings 80 are not located in this area as they may be prone to cracking due to the high stress under the railway rail. To ensure the sleeper has the necessary bearing area, the width of the wings is 240mm. The added benefit from this approach is that the sleeper 10 will hook into the gravel and provide the sleeper 10 with excellent lateral stability. The polymer concrete used to form the railway sleeper 10 is formed with approximately 28% by volume of resin, 22 % by volume of light aggregate and 50% by volume of heavy aggregate. The light aggregate is in the form of centre spheres having a specific gravity of approximately 0.7. The heavy aggregate is formed from crushed basalt having a specific gravity of approximately 2.8 and a particle size of 5-7mm. The light aggregate has a specific gravity that is slightly less than that of the resin whilst the heavy aggregate has a specific gravity that is larger than that of the resin. A thixotrope is added to the resin so that the light aggregate will stay in suspension within the resin. Consequently, the resin together with the lighter aggregate in suspension becomes a flowable filled resin system in its own right. The amount of the lighter aggregate suspended in the resin can be varied as required. To obtain an economical polymer concrete formulation, the lighter aggregate is approximately 45% by volume of the flowable filled resin mix. The heavy aggregate, which is heavier than the resin, sinks to the bottom of the polymer concrete and can as such be positioned in certain parts of the sleeper. By adding the heavier aggregate in specific amounts during the pour, layers or areas of polymer concrete with different amounts of aggregate and hence different density and structural properties can be obtained. FIGS. 2A to 2E shows the process used to cast the railway sleeper 10 using a mould 20. The railway sleeper 10 is cast upside down. FIG. 2A shows the first step in which the polymer concrete 130 is poured in the left and right side of the mould. The polymer concrete is not poured in the middle of the mould. The basalt settles downwardly whilst the resin and e-spheres 131 migrate and also settles in the middle of the mould. The resin and e-spheres form in the middle of the mould 120 form the upper tension zone 100. The basalt is then screeded to the desired level. FIG. 2B shows fibre composite reinforcement members 140 being added in the mould 120. The fibre composite reinforcement members 140 sit on the basalt to hold it in the desired position. The placement of the fibre composite reinforcement members on the basalt causes the level of the resin and e-spheres 131 to rise within the mould. The fibre composite reinforcement members 140 are placed so that no basalt is able to drop through the fibre composite reinforcement members into the upper tension zone 100. Once fibre composite reinforcement members are installed, more polymer concrete 130 is added adjacent the middle of the mould over the fibre composite reinforcement members and also at the ends of the mould, as shown in FIG. 2C. The basalt settles downwardly whilst the resin and e-spheres 131 migrate and also settles in the left and right sides of the mould to form the lower tension zones 90. More fibre composite reinforcement members that extend the majority of the length of the sleeper are placed within the mould as shown in FIG. 2D. The fibre composite reinforcement members are supported by screeded basalt in the middle and at the far left and right of the mould. The placement of the fibre composite reinforcement members on the basalt causes the level of the resin and e-spheres 131 to rise within the mould 120. Extra polymer concrete 130 is poured onto the middle of the fibre composite reinforcement member 140 and the far left and right side of the mould 120 as shown in FIG. 2E. The basalt settles and the resin and e- sphere 131 self level to completed the moulding of the railway sleeper 10. FIGS. 3A to 3D shows the application of a normal loading of a sleeper. FIG. 3A shows the sleeper without any loading. FIG. 3B shows a load that is applied to a sleeper with FIG. 3C showing the tensile and compression zones that are created. The sleeper of the present invention is specifically able to cope with this loading as the sleeper has tension zones that have more resin located within them than in the reminder of the sleeper. The tension zones therefore enable the sleeper to have different load carrying characterises throughout the sleeper to cater for different loads at different points. FIG. 3D shows that the load characteristics in the tension zones can be further improved by placing fibre composite reinforced plastic members in the tension zones. It should be appreciated that the fibre composite reinforcement members may be located at other regions and in different orientations to provide different load carrying characteristics for the sleeper. FIGS. 4A and 4B show a sleeper having top fibre composite reinforcement members 140. FIGS 5A and 5B show a sleeper having bottom fibre composite reinforcement members 140. FIGS. 6A and 6B show a sleeper having a variation of bottom fibre composite reinforcement members 140. FIGS. 7A and 7B show a sleeper having both top and bottom fibre composite reinforcement members 140. FIGS. 8A and 8B show a sleeper having fibre composite reinforcement members to cope with increased shear. It should be appreciated that various other changes and modifications may be made to the embodiments described without departing from the spirit or scope of the invention.