What is the origin of the idiom "gravy train" and when to use it?

The idiom "gravy train" has its origin in the early 20th century, when it was used to describe a train that transported wealthy passengers in luxury and comfort. The term "gravy" was used to describe the delicious sauce that is often served with meat, and it was applied metaphorically to describe the luxurious lifestyle enjoyed by the wealthy passengers on the train. Over time, the phrase came to be used figuratively to describe any situation where someone is able to make a lot of money with little effort.