WHAT IS CLAIMED IS:

1. A turbine driven pump gearbox system comprising: a turbine engine; a planetary gearbox driven by the turbine engine; a primary shaft driven directly by the planetary gearbox, the primary shaft having a first primary shaft gear and a second primary shaft gear; a power takeoff device for generating power when driven by the primary shaft; a secondary shaft having a first secondary shaft gear and a second secondary shaft gear; a collar associated with the secondary shaft for selectively disengaging or engaging the primary shaft and the secondary shaft in at least one of three positions: a first engaged position where the first primary shaft gear engages the first secondary shaft gear; a second engaged position where the second primary shaft gear engages the second secondary shaft gear; a disengaged position, being a neutral position, where the primary shaft is disengaged from the secondary shaft; a primary shaft brake for braking a primary shaft rotation; and a secondary shaft brake for braking a secondary shaft rotation, wherein the power takeoff device, the primary shaft brake, and the secondary shaft brake are controlled as a clutch to shift engagement of the primary shaft and the secondary shaft among the first engaged position, second engaged position, and disengaged position.

2. The turbine driven pump gearbox system according to claim 1, wherein the power takeoff device is configured to run as a motor to control movement of the primary shaft.

3. The turbine driven pump gearbox system according to claim 2, wherein the power takeoff device is a generator configured to generate electricity in a generator mode and configured to run as the motor to control movement of the primary shaft in a motor mode.

4. The turbine driven pump gearbox system according to claim 1, further comprising a shifter in mechanical connection with the collar to move the collar for selectively disengaging or engaging the primary shaft and the secondary shaft in the at least one of three positions.

5. The turbine driven pump gearbox system according to claim 1, wherein a first gear ratio of the first primary shaft gear and the first secondary shaft gear is different than a second gear ratio of the second primary shaft gear and the second secondary shaft gear.

6. The turbine driven pump gearbox system according to claim 1, further comprising a hydraulic fracing pump driven by the secondary shaft, wherein there is no torque converter between the planetary gearbox and the hydraulic fracing pump.

7. The turbine driven pump gearbox system according to claim 1, wherein an electronic controller selectably actuates at least one of (i) the power takeoff device to run as a motor to apply a counter rotation force to the primary shaft and (ii) a primary shaft brake control to apply a braking force to the primary shaft in concert with the shifting of the primary shaft and the secondary shaft between at least two of the first engaged position, second engaged position, and disengaged position.

8. The turbine driven pump gearbox system according to claim 7, further comprising a sensor connected to an outlet of a pump driven by the secondary shaft and measuring a pressure of a fluid pumped by the pump, wherein the electronic controller also selectably actuates (iii) a shifter to shift the primary shaft and the secondary shaft between at least two of the first engaged position, second engaged position, and disengaged position in response to the pressure exceeding an upper or lower threshold.

9. A switchable pump transmission assembly for a turbine driven pump gearbox system, the switchable pump transmission assembly comprising: a primary shaft driven configured to connect to a rotating motive source, the primary shaft having a first primary shaft gear and a second primary shaft gear; a power takeoff device for generating power when driven by the primary shaft; a secondary shaft configured to connect to a rotating motive load, the secondary shaft having a first secondary shaft gear and a second secondary shaft gear; a collar associated with the secondary shaft for selectively disengaging or engaging the primary shaft and the secondary shaft in at least one of three positions: a first engaged position where the first primary shaft gear engages the first secondary shaft gear; a second engaged position where the second primary shaft gear engages the second secondary shaft gear; a disengaged position, being a neutral position, where the primary shaft is disengaged from the secondary shaft; a primary shaft brake for braking a primary shaft rotation; and wherein the power takeoff device and the primary shaft brake are controlled as a clutch to shift engagement of the primary shaft and the secondary shaft among the first engaged position, second engaged position, and disengaged position.

10. The switchable pump transmission assembly of claim 9, further comprising a secondary shaft brake for braking a secondary shaft rotation.

11. The switchable pump transmission assembly according to claim 9, wherein the power takeoff device is configured to run as a motor to control movement of the primary shaft.

12. The switchable pump transmission assembly according to claim 9, wherein the power takeoff device is a generator configured to generate electricity in a generator mode and configured to run as a motor to control movement of the primary shaft in a motor mode.

13. The switchable pump transmission assembly according to claim 9, further comprising a shifter in mechanical connection with the collar to move the collar for selectively disengaging or engaging the primary shaft and the secondary shaft in the at least one of three positions.

14. The switchable pump transmission assembly according to claim 9, wherein a first gear ratio of the first primary shaft gear and the first secondary shaft gear is different than a second gear ratio of the second primary shaft gear and the second secondary shaft gear.

15. The switchable pump transmission assembly according to claim 9, further comprising an electronic controller connected to the power takeoff device and connected to a primary shaft brake control, wherein the electronic controller selectably actuates at least one of (i) the power takeoff device to run as a motor to apply a counter rotation force to the primary shaft and (ii) the primary shaft brake control to apply a braking force to the primary shaft in concert with the shifting of the primary shaft and the secondary shaft between at least two of the first engaged position, second engaged position, and disengaged position.

16. The switchable pump transmission assembly according to claim 15, further comprising a sensor connected to the electronic controller and configured to measure a pressure of a fluid pumped by a pump driven by the secondary shaft, wherein the electronic controller also selectably actuates (iii) a shifter to shift the primary shaft and the secondary shaft between at least two of the first engaged position, second engaged position, and disengaged position in response to the pressure exceeding an upper or lower threshold. 17. The switchable pump transmission assembly according to claim 9, further comprising a pump connected to the secondary shaft and driven by a rotation of the secondary shaft, wherein the switchable pump transmission assembly does not include a torque converter.

18. The switchable pump transmission assembly according to claim 9, wherein the switchable pump transmission assembly does not include a torque converter.