I see that some people get stuck in this map in some areas. Here's a couple of spots that some seem like they're getting stuck on or could get potentially stuck on.

First area just after the train station: Interact with the generator on the side facing the door. It'll unlock it.

Second area with the double teleporters: There's a button on the side of the raised walkway beside where the SMG using Combine soldier is standing. Interact with it and it will unlock the door on the raised walkway. DON'T TAKE THE TELEPORTERS. The mapmaker literally sign posts this with a 'Do Not Enter' sign.

I don't mind if anyone adds to this list with any other areas that people commonly get stuck on.