Green’s Dictionary of Slang

1. one who, while nominally one of a group, team, crew etc, takes no active or useful part in the general efforts.

1853

T. Haliburton Sam Slick’s Wise Saws I 88: You hear a fellow sayin’ sometimes – I’m only a passenger. How little the critter knows of what he is talking, when he uses that cant phrase.

2. (US) a passenger train; thus passenger stiff, a tramp who rides passenger or fast freight trains.

1899		J. Flynt Tramping with Tramps 355: When he rides a ‘passenger,’ for instance, either on top or between the wheels, he encounters numerous dangers.
1907		J. London Road 124: I was riding ‘passengers’ then and making time; but he must have been riding passengers, too, and with more luck or skill than I, for he got into Mission ahead of me.
1931		Irwin Amer. Tramp and Und. Sl. 141: Passenger Stiff. – A tramp or hobo with a mania for riding only the faster freight or passenger trains.
1997		D.H. Edwards The World Don’t Owe Me Nothing 154: [I] blinded a passenger [train] fsrom Chicago to St. Louis .

3. (US prison) a friend [i.e. one who is in the car n. (2)].

2000

Other Side of the Wall: Prisoner’s Dict. July 🌐 Passenger: A friend, someone who is ‘in the car.’.