Green’s Dictionary of Slang

(US Und.) the entrance to a streetcar.

1937		(con. 1905–25) E.H. Sutherland Professional Thief (1956) 41: A cannon mob may be working a get-on (place where many people enter streetcars) and beat the first man just before he steps on the car.
1950		Goldin et al. DAUL 79/2: Get-on, n. (Pickpocket jargon) Any place where crowds enter cars, trains, elevators, etc.