Revealing mistake: When Davey cracks the egg, you can tell he doesn't crack it into his mouth. It slides down his cheek and hits the ground. You can actually hear this happen.
Continuity mistake: In the scene where all the boys are at the junkyard tossing their coins.Watch Chris reach Gordy back his coin, then the next shot Chris hands him the coin again.
Continuity mistake: Watch carefully in the scene where the four boys are running for their lives over the railway bridge. Chris and Teddy are in front, and Vern and Gordie are behind. In one shot we see just Vern and Gordie's feet, and quite a lot of the track behind them. You can see that there is no train behind them. In the very next shot, the train is about two metres behind them.
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.