Abstract: A dual-purpose door system includes a door to be snugly received within an aperture of a vehicle body. The door has the lower end pivotally connected to a slide unit which is mounted upon the body and movable in an outward/inward direction and a sideward direction together with the door for thereby opening and closing the door in an ordinary mode of the system, and the upper end detachably received and guided by a guide unit mounted upon the body. The system further includes an extension board pivotally supported at its one end to the top end of the door and outwardly swingable about the one end subsequent to an outward pivotal movement of the door so as to provide an extended stepping board or path for the passengers in an emergency mode of the system. There is provided a buffer for absorbing objectionable pulses or shock loads which may be impressed upon the door and the board when they are opened in accordance with the latter mode.