EMPLOYEE WAS RIDING IN THE FRONT AREA OF A BATTERY POWERED RAIL RUNNER MANTRIP WHEN IT JUMPED RAIL CAUSING THE SUDDEN IMPACT OF THE FRAME OF THE MANTRIP AGAINST THE TOP OF THE RAIL. THE SUDDEN JOLT FROM THE IMPACT CAUSED A BACK STRAIN AS HE WAS SITTING IN A CROUCHED POSITION WHEN THE ACCIDENT OCCURRED.