Taking on a "loon" whacking away with a hammer, unarmed and by yourself, could be tricky. My guess would be a good kick or three in the groin would be a good way to start. But fighting in a subway is a nasty place. I don't have any actual experience in subways, but have gone a few rounds with fighting drunks on a bus, and it is real tight quarters, hard to get a good swing with either foot or fist...