I agree with Earl - especially since you say it was your elbow that was bleeding. There is nothing on the vehicles that you could scrape your elbow on during the ride.
It also doesn't sound like your experience hurt much. If, in the middle of the ride, you suddenly felt pain in your elbow - I would have guessed that you got hit by something a guest threw at the train as you went under the bridge (yes, it happens, and yes, that's why Universal has that entire area videotaped).
From your trip report, I would venture to say you cut your elbow on your locker (did you use one?) or something in the queue - particularly since you used the singles line (chain link fence, etc).