The installation hereafter of platform elevators as defined in these rules shall not be permitted. Existing platform elevators may be permitted for travel not to exceed 50 feet between terminal landings. Hoistways for such platform elevators shall be guarded in the same manner as hoistways and platforms for freight elevators. The sides of the platforms shall be enclosed in the same manner as required for freight elevator cars. No person shall be permitted to ride on such elevators, and signs to that effect shall be posted on the enclosure.