Abstract: The invention provides a door system for a transit vehicle. It has an actuator which provides a rotary motion about an axis of the actuator for moving the door panel between an open position uncovering the aperture and a closed unlocked position covering the aperture. The actuator also provides a linear motion along the axis of the actuator for moving the door panel between the closed unlocked position and a closed locked position. The rotary motion followed by the axial motion is provided by a cylinder having a piston moved by fluid pressure. The piston includes a guide track in its exterior which engages rollers in the cylinder. The guide track has one portion curving around the piston and one portion parallel to the axis. The curving part provides the rotary motion and the straight part provides the axial motion. When the rotary motion is completed the piston encounters a stop on a shaft which supports the door. The actuator then moves the door axially to engage locks.