A gravy train is a job that is paying you more money than the work is actually worth. Therefore, you are not just receiving your meal; you are receiving gravy on top of it, as a metaphor for additional, surplus pay. Ex.: "That secretary's on the gravy train. She's being paid way more than she's worth because she's having an affair with the boss!" Figuratively, you ride the gravy train as far as it will take you.