Gravy train - any easy and lucrative means of obtaining money
By the beginning of the 20th century gravy was being used to mean money that had been easily acquired, or that was extra in some way, such as a bonus or tip, in the same way that gravy is an extra on top of the basics of a meal. It could also be used for money obtained through extortion or other illicit means. A gravy train was US railroad slang for an easy run where the pay was good. This was adopted into general speech in the 1920s.
