Abstract: The present invention is a cartridge bridge plate assembly for mounting under a floor structure of a transit vehicle adjacent a door opening. It has a frame assembly and means for attaching the frame assembly underneath the floor structure of the vehicle. It has a bridge plate for covering a gap between the floor of the vehicle and an adjacent surface to facilitate passenger ingress and egress when the vehicle is stopped. It has a bridge plate positioning means for positioning the bridge plate and accommodating motion of the bridge plate from a stowed position within the frame assembly to an extended position for covering the gap. A motor is connected to the bridge plate positioning means for moving the bridge plate from the stowed position to the extended position and from the extended position to the stowed position. The system includes a power connection to the motor, and a drive screw connected to and rotated by the motor.