This is not a problem in practice, since there is enough spare traction power available that the train can continue on its journey, on three packs out of four total. There is no collector as on DC motors, which allows a reduction of wear and maintenance costs. The final drive is a gear train that rides on the axle itself and transfers power to the wheels. This final drive assembly is restrained from rotating with the axle by a reaction linkage. Schematic of the TGV's tripod transmission.
Sensors continuously compare motor speed to axle speed.
A discrepancy between the measured speeds indicates a tripod driveshaft failure condition, which is indicated in the cab. If the tripod transmission develops a dangerous vibration, the shaking reaction linkage can strike a pneumatic valve that automatically dumps the main brake pipe and stops the train. Other Components Besides the main traction chain, there is of course a whole suite of auxiliary equipment inside the TGV power car. Overall they can take up almost half of the braking energy in stopping a trainset from full speed. There are two sets of rheostats, one for each power pack.