Similar to road head, train brain is getting a blowjob on an extremely crowded train.
Carl: Dude, I got the sickest train brain yesterday.
Chad: Nice, what happened?
Carl: I ended up cumming on the guy standing next to me and getting kicked off the train.
Chad: Gnarly.
When a girl gives a blowjob to more then one guy at the same time. The girl is the locomotive pulling the other cars of the train. thus called a brian train.
"last night we had a brain train going with this girl.. she was blowing all three of us at the same time!"
"Shit yo, your brain train just derailed! That shit ain't germane to ANY topic" - said to someone whose conversation went from sleeping bags to whether it would hurt to be slapped in the face with a dick