Of American-English origin, gravy train denotes a situation in which someone can make a lot of money for very little effort.

It originated in the use of gravy in the figurative sense of advantage, benefit, also of American-English origin; the earliest instance that I have found is from the Fayetteville Observer (Fayetteville, North Carolina) of 9^th April 1845 (although the subject of this paragraph is unintelligible to us, the image is clear — the Globe, the Constitution and the Madisonian were newspapers):

“The Organ.”—The Globe publishes the list of letters, whilst the Constitution publishes first the official notices from the Treasury Department. The division of the spoils here is like that described in the farce. “Domini Felix gets the meat, and Lingo gets the gravy.” The Madisonian gets nothing.

The second-earliest instance that I have found is from The Weekly Kansas Chief (Troy, Kansas) of 18^th February 1875:

It has heretofore been the custom, in State elections, to pay out of the funds raised for campaigning purposes, a certain amount to each newspaper of the party, to help pay for printing tickets, announcing meetings, and similar work. But last year, this fund was divided up among the city papers—but very few, if any, of the country papers receiving any thing. Yet the country papers elected the ticket. The few that bolted, damaged the ticket immensely; had a majority of them done so, the defeat would have been overwhelming. Party principle is a good thing; but those who get the gravy, should pay for it.

The earliest occurrence of gravy train that I have found is from The Florence Herald (Florence, Alabama) of 14^th September 1899, which reported on an improvised friendly baseball match between Tuscumbia and Florence; here, therefore, gravy train does not refer to financial gain but to sporting achievement:

From the outset it was evident that Florence was on the “gravy train,” as some of the boys afterward expressed it, and they had no trouble at all in administering a very crushing defeat to the visitors.

The second-earliest instance of gravy train that I have found is from The Alexander City Outlook (Alexander City, Alabama) of 12^th October 1900:

If the McKinley¹ administration is responsible for the “full dinner pail” throughout the land, we suppose Your Uncle Hanna² has full charge of the “gravy train.” Those patroits [misprint for patriots] who are on the hog will please take notice.

¹ William McKinley (1843-1901), President of the United States from 1897 until his assassination in September 1901
² Marcus Alonzo Hanna (1837-1904), American businessman and Republican politician, who used his wealth and business skills to successfully manage the presidential campaigns of his friend McKinley in 1896 and 1900