The “Game of Thrones” gravy train is still rolling over at HBO . . . so obviously, they’re trying to figure out how to run it until the wheels fall off. Here’s an idea: A PREQUEL SERIES.

They’ve already approached GEORGE R.R. MARTIN about it. One exec says, quote, “I would say it’s still kind of preliminary ongoing talks. There are areas we are exploring.”

Meanwhile, Season 7 of “Game of Thrones” doesn’t start until next summer. It’ll have seven episodes.