PURPOSE: To improve the loading efficiency and the operation convenience by designing the truck shape to be of cab over type, presetting the width of the truck and the roof to be within the specified range of the railroad vehicles, and mounting the stay to fit a side mirror in a flexible and rotatable manner.

CONSTITUTION: The shape of a truck 2 is of the cab over type, and the truck width W is the maximum width where the truck can safely travel on the railroad track. The dimensions a, b, c of a roof 11 are set in the specified range for the railroad vehicles, and the roof is formed in the shape of the Quonset hut. A stay 101 where a side mirror 12 is fitted is turnable in the direction of the arrow (d) around the supporting point 102, and flexible in the direction of the arrow (e), and can be inserted into the body in a retreative manner. This constitution provides the truck for piggy back transportation system with good loading efficiency.