Picadilly Underground Line
Took the Picadilly line to and from Heathrow Airport, as well as around parts of central London. The subway cars are much narrower than those in Toronto (which I am more familiar with). There is just enough room to move between two seats of benches facing each other.
You can get an idea of the size of the subway cars from this intriguing Youtube video of "Thriller" being performed in the moving cars of the London Underground.
Or for something on a more classical theme, here is a video of a "dinner party" on the tube. www.youtube.com/watch?v=ywLhNwSBizE