How Do You Handle a Hungry Man? (Harpers.org): "Weldon looks to be especially generous in using campaign money to treat himself to dinner, but he's not alone. The American Prospect recently found another Pennsylvanian, Republican Senator Rick Santorum, spending political cash for a host of suspect items, including 66 trips to Starbucks, mostly in his hometown of Leesburg. Of course, neither Weldon, Santorum, nor their peers seem worried about making liberal use of campaign donations, and that’s because the Congressional Ethics Committees (or the “Member Protection Committees,” as one watchdog once described them to me) are not about to stop the gravy train."
No, not about the politicians (no surprise there). About the phrase. Well wonder no more. According to the Online Etymology Dictionary "gravy train (1927) was originally railroad slang for a short haul that paid well."