Green’s Dictionary of Slang

gravy train n.

[gravy n. (2) + SE train; Dillard, Lexicon of Black English (1977), suggests the image of a gambler who is in the ‘gravy’ and thus pursued by a ‘train’ of those who wish to benefit]

[late 19C+] (orig. US) a sinecure, a simple, substantially profitable situation from which one can benefit easily; thus ride the gravy train

In phrases

ride the gravy train (v.) (also ride a gravy train, ...the gravy boat)

[20C+] (US) to enjoy a comfortable life; thus gravy-riding n.