Just before the train hits the car, Clear and Billy are standing towards the front end of the car. and Alex pulls Carter out of the car towards the back end of the car. So, they should have been on opposite sides of the railroad tracks. But after the train hit the car, they're all together on the same side of the train.
It didn't show then fall to the rear of the car. All it showed was them falling on the ground and looking back at the train as they scooted backward. Plus if you notice the train was clearing the screen to the right and you could see them under the train moving backwards. This would mean they were in front of the car, not behind it. Make sure you know what you're talking about before you add a slip-up.
No they do fall behind the car. Sitting in the car, with the train coming at them the train is moving to the right, towards the driver's side. When the guy gets his head cut off the train is moving right so they are behind the car.
With all these slip-ups I still think that this is really a great movie because with the plot and everything, it's complicated, they made everything up.