PURPOSE: To vary a gauge while traveling by locking/releasing shaft box side locking members on/from locking members arranged on a pair of left and right wheel shaft outer cylinders movable in the shaft direction, and respectively meshing torque transmitting cylinders with a pair of left and right sliding gears arranged on a driving transmitting large gear support cylinder close to a driving motor.

CONSTITUTION: Left and right wheel shaft outer cylinders 17 and 17 are movably fitted around a wheel shaft 7, and locking protrusions 21 and 22 are arranged on outer peripheral upper surfaces of locking blocks 20 and 20 fixed to its outside, and locking grooves 23 on upper surfaces of the inner periphery of respective shaft boxes 6 are engaged. Release of its engagement is performed by relatively moving the shaft boxes 6 in the upward direction to the wheel shaft outer cylinders 17 in a lowering inclined area of a traveling rail. A large gear support cylinder 27 is protrusively arranged on a large gear to transmit output of a driving motor 8, and sliding gears 30 and 31 are formed on its left and right inner peripheral surfaces, and sliding gears 30 and 34 mesh with each other in a left side torque transmitting cylinder 32 in the wheel shaft 7, and sliding gears 31 and 35 mesh with each other in a right side torque transmitting cylinder 33. In this way, a gauge is varied.