(To Be on the) Gravy Train

What does the idiom “(To Be on the) Gravy Train” mean?

Definition:

To make an easy living, to benefit easily from one’s association with something that brings profits

For example:

“Ever since her company’s stock split, Rita’s been on the gravy train—she was given stock as a benefit.”

Notes:

This expression probably originates from the language of railroad hoboes.