The child's grandmother found herself trapped on the tracks after she tried to drive over a level crossing while warning lights were flashing.
Barriers fell before she had reached the far side, with one barrier slamming down on her car boot.
She leapt out of the driver's seat - leaving her grandson strapped inside - and tried to lift the barrier. Before she could free her car, a commuter train slammed into it - ripping off the front end and spinning the vehicle around.
Amazingly, both the baby and his gran escaped unhurt in Roy City, Utah.
Local police chief Roy Whinham urged drivers to 'keep going' if they find themselves trapped on the tracks.
He said: "The design of those crossing arms is that you drive through them. You get yourself off the tracks, whether you go backward or forward, it doesn't matter. And we don't care if you bust the arms off. They're actually designed to break away with very little pressure."