Nobody, just nobody talks to strangers on the tube (this includes platforms, escalators, ticket offices).
I learnt from bitter experience that anyone trying to interact either wanted money or was a weirdo.
Its like with chuggers, you have to not make eye contact!
I'm sure this is probably true in all big cities on public transport. People are starved of personal space and over stimulated, they do not want randoms getting in their face.
Im glad I don't live there any more, its too bloody stressful. But going on a one person crusade to speak to every person I encountered daily (WTF, seriously?!) wouldn't have made it more pleasant, it would just have made me a weirdo magnet.
Its like that film, I think it's called Enchanted, where snow white wakes up in new York and tries to call the woodland creatures to help her tidy up - she's appalled that all she gets are mangy rats and pigeons!!