Green’s Dictionary of Slang

passenger n.

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

[US]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.

[US]J. Flynt Tramping with Tramps 355: When he rides a ‘passenger,’ for instance, either on top or between the wheels, he encounters numerous dangers.
[US]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.
[US]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.

3. (US prison) a friend [i.e. one who is in the car n. (1)].

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