Green’s Dictionary of Slang

flipper n.5

[flip v.1 (2)]

(US tramp) a tramp who rides the railroads, rather than travels by road.

[US]Irwin Amer. Tramp and Und. Sl. 76: Flipper.– [...] A tramp who ‘flips’ or rides trains, as opposed to the ‘dyno’ who uses the highways to get about the country.