I was on the tube with my friends once, going into London. I let them direct me, I was otherwise engaged holding myself closed. We'd been drinking beforehand, but I wasn't too drunk, I just needed a piss really really badly. We got to Holborn, and they said "time to get out", so I did. Just as I thought we were about to get out of the station into a good open public place to find a suitable place to have a good one, and they told me to get on another train.
I was bursting. :>.<: I can quite easily see how it's possible to wet yourself on the train if you're that desperate. Just hope it never happens to me...