Abstract: An emergency braking device for railway cars having a wheel truck provided with a fluid pressurized brake line in a wheel braking system, comprises an elongated tubular break-away valve member rigidly attached to one of the truck and car frame, and is connected in the pressurized brake line. A flexible substantially inextensible cable is connected between the valve member and the other of the truck and car frame. The cable is connected between points which are on a line substantially normal to the valve member. The cable length is such as to permit normal movement between the car frame and the truck without being tensioned but is tensioned during abnormal movement therebetween corresponding to an emergency condition, at which time the valve member is caused to rupture providing an escape path for the pressurized brake fluid to automatically brake the railway car.