Here's a news item about MEPs (Members of the European Parliament) being on the gravy train.
You might expect them to travel by train but how does this collocate with gravy?
Gravy is a liquid made from meat juices that you pour over meat to make it extra yummy!
The gravy train is not one made of gravy but a metaphor for the high life. It causes a lot of anger in Britain when people find out their tax money is being used to support a luxury lifestyle.
By the way, don't confuse gravy train with gravy boat - that's one of these and is used to pour out the gravy.
Finally, why not find out how to make gravy. No train needed.