EPLA is the electro pneumatic valve, electro pneumatic braking force transformation as the international general switching control, belonging to a control valve, and its role is when braking, brake controller through a braking command, load signal to calculate the actual needs of the air braking force, the braking force is converted to current EPLA valve, with the current produced is proportional to the magnitude of air pressure by EP valve switch, thereby controlling relay valve supply / exhaust, the air pressure can realize continuous stepless regulation.