Metro station managers and security folk are usually beyond anal about not letting you photograph in stations. I once got yelled at for taking artsy photos with a digisnapper of 1 second exposures of people's feet as they were passing through the turnstiles. No faces, just floors, turnstiles, and feet. I think your best bet is to try and arrange permission beforehand, otherwise you will at best be made to feel like pondscum for trying, and worst case, you will get hauled off and your gear confiscated, of which at least part you can guarantee you'll never see again thanks to the sticky fingers of the folks at Central Booking.