I would want some tool that could help me escape from a stuck subway train, eg if there was an underground accident or it just stopped due to a power cut or something. Something that could break the windows, probably.
Learned this one years ago...
One of the best tools (for men) when riding a subway or elevated transit system is an empty 1L (qt) water bottle and a coat/sweater for coverage.
Sometimes the train gets stopped outside a station for an extended period of time (1-1/4 hours is the record for me). I think most here can figure out the rest of this...