§ 239. Trains to come to full stop. All trains upon elevated railroads shall come to a full stop before any passenger shall be permitted to leave such trains; and no train on such railroad shall be permitted to start until every passenger desiring to depart therefrom shall have left the train, provided such passenger has manifested his intention to so depart by moving toward or upon the platform of any car; nor until every passenger upon the platform or station at which such train has stopped, and desiring to board or enter such cars, shall have actually boarded or entered the same, but no person shall be permitted to enter or board any train after due notice from an authorized employee of such corporation that such train is full and that no more passengers can be then received.