Doom 3 - Monarail ?

I'm playing Doom 3 and I'm at the end of Monarail and I've killed all the bad guys (I even put it in GOD mode) took a look at the cheat sheet and I still can't figure out how to get to the next level can anyone help?

How about looking at a walkthrue!

See if this helps

Or this.

watch the 'You tube' video