After you finally get out of Ravenholm, you will go through a tunnel-type area with tons of Headcrabs, past the mine cart with the big spinning deathtrap, and finally see a light at the end of the tunnel. You will exit into a train station. Normally, you would go all the way down until some crashed trains block you from going further, forcing you to go left into a fenced area full of combine. Enable the "noclip" code and walk through all the crashed trains. You come out in an open area with tracks. In the middle of the tracks is G-Man with his briefcase. Regrettably, you cannot talk to him or do anything else in this area.