To provide a slope speed control method for a railroad freight car capable of maintaining a speed of a train within a narrow range even on a downward slope.

This slope speed control method is one for a railroad freight car provided with a locomotive and at least one connecting car. The coupling car is provided with an ECP brake control system provided with a brake cylinder device driven by hydraulic pressure and a step release valve. This method includes a step of monitoring actual train speed, a step of selecting a target train speed, a step of detecting a speed difference between the target speed and the actual speed, and a step of responding to the speed difference and meeting the actual speed generally with the target speed by adjusting the brake cylinders of at least one coupling cars.