Other mistake: Teddy's hair changes in every shot after being dunked in the swamp. The strap to his bag disappears and reappears as well.
Other mistake: In the treehouse scene, Teddy tells Vern that nobody will care if they didn't go to the drag race. When Teddy says this you can see Chris mouthing his lines. (00:07:45)
Answer: No, it wasn't that big of a train. He didn't even attempt to get off the throttle. That's all it would have taken for the boys to make it fairly easy. It was a straight-away track, no chance of it derailing by hitting the brakes. Like the man said above, if trains derailed that easily, we wouldn't be using them.