The Polar Express is a Christmas movie classic, but did you know it features an incredible Back to the Future Easter Egg?

The reason is that both movies were directed by Robert Zemeckis, who inserted that blink-and-you’ll-miss-it nod.

Film fans will remember at the end of Back to the Future Part III when Doc Brown and his family fly off in their steam-powered time-travelling train.

Well, in The Polar Express, the titular locomotive also appears to be a time machine.

We know this because a flux capacitor is spotted in the Polar Express’s cab when the cotter pin lands and bouncers off the hat of driver Smokey, one of the train’s twin engineers.

In Back to the Future, the flux capacitor is the device that came to Doc Brown’s mind on November 5, 1955. On that fateful day when Marty McFly arrived back in time, the eccentric inventor slipped and bumped his head after falling off his toilet while attempting to hang a clock. That’s when the device came to his mind and he essentially invented time travel. Therefore, the fact that the Polar Express has a flux capacitor helps explain why it appears to run on its own time.