Gravy train

from where was this term derived?

GRAVY TRAIN - "In the 1920s, railroad men invented the express to 'ride the gravy train' to describe a run on which there was good pay and little work. The words were quickly adopted into general speech, meaning to have an easy job that pays well, or, more commonly, to be prosperous. 'Gravy,' however, had been slang for easy money since the early 1900s." From Encyclopedia of Word and Phrase Origins by Robert Hendrickson (Facts on File, New York, 1997).