A prime mover of an underground mining machine is propelled by motor driven endless tracks that are operated in unison or independently to effect forward and reverse movement of the machine and turning of the machine. Drive is transferred to each track from a motor mounted in a traction case on each side of the machine to an input shaft through first and second stage planetary assemblies to a sprocket assembly that rotatably supports the respective tracks. A planetary carrier of the first stage planetary assembly is longitudinally movable under a spring force to apply a braking force through a brake disc to a brake plate extending from the input shaft. Fluid pressure in the traction case opposes the spring force to normally maintain the planetary carrier removed from braking engagement with the brake disc. In the event the fluid pressure drops below a preselected level, the spring force moves the planetary carrier longitudinally to actuate the brake disc assembly to interrupt rotation of the input shaft and brake the vehicle.