PURPOSE: To effectively cool a device, by forming a ventilating hole for running air to pass, on the bottom plate section of a frame fixed on a truck frame.

CONSTITUTION: The truck frame 11 of a car body is designed such that the upper and lower hearing sections 12, 13 are respectively of one-piece plates, and both the plates are connected to each other with a vertical center sill 14 between the plates, and are opened to the advancing direction of the car body, and running air flows in the front and rear directions. A frame 15 is of a 'U'- formed cross-sectional area opened on the rear side of the advancing direction, and a stationary section 17 extended to the rear side from the upper end of the bottom plate section 16 of the frame 15 is fixed on the upper bearing section 12 of said truck frame 11 with a bolt 18. On the central section of the bottom plate section 16 of the frame 15, a ventilating hole 20 is formed, and at positions confronted with the internal side of both side plate sections 21, pole plates 22 are circularly arranged and fitted, and at the plates 22, a brake disc 24 is positioned. Then, the running air flows into an internal section from the ventilating hole 20, and the magnetic pole 22 and the brake disc 24 can be cooled.