§ 240. Gates or vestibule doors. Every car used for passengers upon elevated railroads shall have gates at the outer edge of its platforms so constructed that they shall, when opened, be caught and held open by such catch or spring as will prevent their swinging and obstructing passengers in their egress from or ingress to such cars, or vestibule doors so constructed as to slide into the body of the car; and every such gate or door shall be kept closed while the car is in motion; and when the car has stopped and a gate or door has been opened, the car shall not start until such gate or door is again firmly closed.