Ah you took the wrong route after the gate. See there's two objectives, one being the rock clearing with the tnt (which you did) or to uncouple the wagons chain (which was just a bit further down the tunnel), and seeing as you dropped down with the tnt objective(which you can still do even if it's the wagon objective, for instance I'll do that if I'm far behind other players who are ahead of me to catch up faster) you were stuck down there without being able to get back to the wagon to uncouple it. I'm sure you'll get it next time though. Ever need any help on maps feel free to leave a comment on steam, I go by the same name. Or find me in game, I admin on quite a few servers so I hop around a lot and also just hop on random servers quite often.
Servers I admin on include Zans servers (one objective only and the other mixed)
The server called ??? (classic/realism/nightmare)
Vertigo gaming servers.
Lowkey.
And the 1upz servers both nightmare and realism.