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The Citizen app gives minute to minute accounts of subway fatalities, you can learn a thing or two from it. In most cases that I've followed, it takes 30 minutes to a few hours. I imagine if your head gets run over by the wheel directly it would be quick but usually it's something a bit less critical, like say your limbs wrapped around an axle or your body wedged between a bogey and the ground and dragged for 20 meaters or so leaving a trail of gristle and textiles and hair. Then the train stops and they shine a flashlight at you and if it looks like you're not completely dead, the FDNY comes and tries to pry the train off you with powerful equipment, at which point you're probably going to die when all your organs and blood come out.
Probably getting spattered by a shinkansen would be painless and quick but those tracks are well protected