Green’s Dictionary of Slang

(US Und.) a tramp who rides the platforms of passenger or freight cars, the tender of an engine and similar spaces.

1916		‘A-No. 1’ Snare of the Road 32: ‘Catters’ [...] ride the platforms of the express and baggage cars, the tenders of engines, and similar places.
1931		Irwin Amer. Tramp and Und. Sl. 48: Catter. – A tramp or hobo riding the ‘blinds’ or on back of the engine tender, in which position one must hang on for dear life and keep on the alert to prevent discovery.