The painful slap in the face one receives when they lean too far over the subway track to see if the train is coming. The subway comes... and they are slapped in the face by the train. Not hard enough to knock off their head, but just hard enough for it to be considered a Train Slap.
Me: " You better step away from the tracks, Katie, you're in for a painful train slap."
Katie: " I've been train slapped before, and I won't let it happen again"