Low-load start-up hydraulic system for controlling rotation of a railroad car body

A hydraulic system (1) for controlling rotation of a railroad car body (2), and presenting a motor pump assembly (8) connected to a hydraulic fluid tank (7) and to a supply line (14); a number of hydraulic actuators (3a, 3b, 4a, 4b) for exerting mechanical force on the body (2) to rotate it; a solenoid valve (26) for selectively connecting the hydraulic actuators (3a, 3b, 4a, 4b) to the supply line (14) or to a drain (29) in response to electric control signals; and a bypass start valve (64) for depressurizing the supply line (14) during start-up of the motor pump assembly (8); the bypass start valve (64) being a purely hydraulically operated, normally-open valve, which switches automatically when supply from the motor pump assembly (8) reaches a predetermined threshold value.