You’ll see a bit more of the Martian surface here. I guess the claim that idtech4 can’t do large outdoor areas was false even back then. Anyway, the train ride is pretty fun although it’s purely cinematic, there’s not much interaction until you arrive at Site 2.
Now here is where they made possibly the single biggest blunder in Doom 3. Opening the monorail airlock (the only real objective in this level) requires a code, which can only be found by reading the emails on the PDA. Why they didn’t simply make this a button is beyond me, as the code doesn’t really add anything. So this is where the game does force you to read the stuff on the PDA. It’s not localized, and I’ve watched a Let’s Play by a German player who didn’t know what to do here. It sucks if you don’t speak English, but in this case, it wouldn’t even have been necessary if they just made it a button instead of a number pad.
Once you do figure it out, you can ride the train until it crashes into another malfunctioning airlock. Get off the train, watch imps be mowed down by a ceiling turret, deactivate said turret (this time, it’s a button!) and welcome to the Delta Labs.