I was recently an extra in a short film set on the Underground and spent several hours travelling up and down the Piccadilly Line with cast and crew, trying to shoot a few seconds of HD footage on a Red One.
During the shoot I tried to observe and learn as much as I could vicariously. This is an extremely difficult film environment - possibly more so for the professional using heavy, complex kit like the Red One. Firstly the trains rock and roll and vibrate heavily. This means that, at best, every shot looks like a hand held shot. At worst, particularly on close-ups (and remember, a major USP of the Red One is it's ability to zoom beyond other HD formats) there's too much movement. Given the size of the Red One, some shots could only be achieved with three people holding it steady.
Secondly, there are huge differences in light levels between tunnels and stations, and between underground and overground track. This means you're always up against the clock - you may have only a few minutes to switch lenses, position the camera and crew, and capture that shot before the conditions change.
Thirdly, watch out for continuity. Each time the train passed through central London and came out of the tunnels, we had to change trains and head back the other way, finding a similar position in the new carriage, and of course the minor details such as the posters above the seats had all changed.
The general public weren't a difficulty: people were happy to move along the carriage to let us film at one end. I suppose you could shoot a scene like this without extras, using volunteers as you go along - the problem would be with the train changes or re-takes as there would be different people in the background of each shot. Overall the shoot was good fun and I was impressed with the team's ability to rise to all these challenges and really hope they got what they needed.
If you can deal with it's idiosyncrasies, the Tube is a fantastic set. The winner of SFL-48 in 2008, Factory Farmed, and the 2009 runner-up, Glimpse, both relied heavily on Jubilee line footage. If you want to see a feature made Underground, watch the 2004 horror Creep. In addition to a superb performance by Franka Potente, the film makes great use of the Underground's hidden treasures - miles of sinister disused stations and tunnels.