I definitely do not condone the act of urinating in public places, but from a male perspective, I at least understand it; its easy and can be done with a relative amount of discretion. Defecating in public transit system, however, requires more cojones than the lowly public urinator. I've never had the unfortunate experience of stumbling upon someone in the act of 'performing', but have stumbled upon the aftermath more than occasionally.
Some subway stations tend to be more notorious than others; the Chambers Street Station (J,Z trains) in Lower Manhattan, in my humble option is one of the worst, with an aroma that resembles an under serviced port-o-let in the middle of July with the combination of bleach (and sometimes without the bleach).
I have stumbled upon other acts of foul Subway Douchery that I suppose deserves honorable mention here: such as people stripping down (and I mean completely) and changing clothes on the station platform or the Saturday night sinner beginning his Sunday redemption early in the form of purging his gastric contents (if I'm lucky, between his own feet, if not, between mine) on a moving subway car. Ahhh, haven't I made your Friday morning complete? No need to thank me ;)