I think I'd have to agree with Whitebird - the train station is a great set-up scene, everybody minding their own business. I spent ages just bumping into metrocops and running away to have them chase me with their stunsticks. That, and picking up every single telephone handset I could find, and...
Well, in the beginning of DX2, even the terrorists have that nanotech bomb they use to completely obliterate Chicago (Was it Chicago? Or Seatlle? Something like that). I reckon something like that would at least take the Combine by surprise when they start trying to shove citadels through portals.