Ugh, I had the same thing happen once, as a tourist. Walked into a subway car with my buddy thinking "look how dumb everyone else is, this car is practically empty!" Doors close, train starts moving, and the stench hits. There's a very dirty homeless fellow at one end of the car with feces sliding down his leg. I could see it at his ankle because his pants were ripped open. Got off and changed cars asap.