The train looks pretty cool, but unfortunately I don't see how this is going to be playable in the slightest bit. It looks like a fun map to mess around in but it's not suited for cp at all. There's only one path, and its extremely narrow! Can you imagine playing this in 8 vs 8, or 12 vs 12, or God forbid, 16 vs 16? That's what I call a clusterfuck.
This is a cool idea though, and I think you should expand upon it. Make more paths. If the train is moving, then perhaps you could have an additional train running on a parallel track, allowing players to jump from one train to another. Give access to the roofs, adding at least another dimension to the level. Think ctf_convoy, only the trains are MUCH closer to each other and nowhere near as big of course.
Otherwise, you can always make the level based around a depot where the train is parked, making the train only a part of your level.
Or why not both? make it a 2 or 3 stage cp map. Stage 1, you have to capture the point at the front of the train, but you can jump onto other moving nearby trains for alternate routes. Stage 2, the train parks at the depot and you need to make your way into the enemy hq through the depot.