After the landslide Vesser assumes the engineer is dead because he's sees his cap lying on a rock near the river. But the cap is yellow and in the scenes where the engineer is driving the train you can clearly see he's wearing a black cap, not a yellow one. The engineer is still dead though.
Actually the 'yellow' hat is the yellow lining of the engineer's hat.
When the train gets to Parkdale and the men get out to take back the depot one of the men is shot by another man in the depot. This man collapses on the tracks. In the next shots the man has disappeared.
Just before the crash, the fireman in the occupied train spots the oncoming train. The engineer tries to slow down. A third man appears and runs back to uncouple the occupied cars. When he gets back to the engine, there are now four people in it. Eventually five people jump off the engine.
When the train slams into the landslide the image is of the train moving along and then suddenly stopping. But this is just the film itself being skipped a few times and then being stopped to display one frame to give the illusion the train had run into something. This is obvious because the movement of the wind blown tree branches and flowing of the river completely freeze when the train comes to a stop.
When the two trains are on a collision course Vesser tells Gil to stop the train and he does; he pushes back on the throttle and applies the brakes. But the train doesn't stop or even slow down. In fact it looks as if i goes faster.
When Vesser jumps off the train to take back the depot Texas Creek one man following him can clearly be seen dropping his rifle on the ground and another man can be seen in the background loosing his footing after he's hopped out of the train car resulting in him falling flat on his face. In the end Vesser was really the only character in the shot who didn't mess up.