Here's what I think happened.
He did climb the whole thing himself, but so did the cameraman, so it's not too impressive.
When he threw the chain up there, some crewmembers were already on the top to secure it safely on the third try.
The trainpart was planned and the train wasn't in the tunnel when they were, after they had run out of the tunnel, the train started passing through and he stood in near the exit until the train was close, at which point he jumped out, then they edited it so that it looked real.
The cameraman wouldn't have been able to jump onto the train, so the it probably stopped for him to get on before Bear did it.