Abstract: A pneumatic piston and cylinder assembly is preset by means of a 3-way regulating valve to exert a balancing force on a blockade arm in a retracted position. The arm is coupled via a linkage to a cam follower which rides on a rotary cam. The arm pivots in response to motion of the cam. When the cam causes the arm to pivot away from the retracted position to an advanced position, the arm overcomes the preset balancing force and causes the piston to retract gradually as air is bled to atmosphere from the cylinder via the valve. At the end of the stroke of the arm, the diminished air pressure in the cylinder provides a cushion to prevent bounce of the arm. As the cam causes the arm to reverse its stroke and pivot back to the retracted position, air is gradually admitted to the cylinder via the valve, thereby extending the piston and assisting movement of the arm.