Hoistways for freight elevators provided with trap doors capable of sustaining a load of not less than 50 pounds per square foot when closed, and so arranged as to open and close with the passage of the car, will be accepted in lieu of the enclosure herein required, provided that in addition to such trap doors the hoistway shall be adequately protected on all sides, all floors, with a standard railing as provided in section 8-1.54, or enclosure at least 42 inches high; on sides used for loading and unloading, self-closing gates not less than 42 inches high shall be provided.