PURPOSE: To prevent derailment, by providing a roller on one end of an arm swingably fitted on a truck movable on a freight car rail and engaging the roller with a wheel of freight car and connecting the arm to an expansible and contractible support.

CONSTITUTION: Supposing the freight car is conveyed rightward in Fig. and the truck 6 is driven rightward at a freight car conveying speed, a wheel 2 of th freight car of lower speed than the truck 6 comes into contact with the right side of roller 3 and a truck driving force is applied to the wheel through the roller 3 to accelerate the wheel. On the other hand, the wheel of the freight car incoming at a speed higher than the truck driving speed comes into contact with the lift side of the roller 3 and a decelerating force is appled to the wheel 2 through the roller to decelerate the wheel. When an excessive force is caused between the wheel 2 and the roller 3, a buffer 8 is compressed, the ar 4 is turned leftward and the wheel 2 passes over the roller 3. Therefore, derailment is prevented