The phrase "Gravy Train" is most often used in the context of expressions such as:

"Now Bert's got an expense account, he's really riding the gravy train"

This suggests that the subject is 'on to a good thing', i.e. making good money for little real work.

Despite what you might think, this phrase does not come from the world of politics or estate agents. The origin of this expression lies with US railroad workers in the 1920's, who used it to describe getting well paid for working on a relatively short or easy stretch of track - they literally 'rode the gravy train'. The expression itself derives from the American use of the word gravy to denote 'easy money'.

See also: gravy