gravy train

Example: “Once the grant money ran out, the gravy train was over.”

Origin: American railroad slang from the 1920s, originally naming a train route that was short, easy, and well paid — a soft assignment rather than a hard one. "Gravy" itself had meant easy or unearned money since the early 1900s, likely because gravy is the part of a meal added on top of the meat rather than anything essential; one further theory traces the word back through Old French grané, meaning made with grain or spices, misread at some point into Middle English gravé. "Ride the gravy train" broadened from railroad slang into a general phrase for landing an unusually easy, profitable situation.