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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.
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A person with autism-spectrum disorder, especially as indicated by narrow, obsessive interests (trains, dinosaurs, Minecraft, etc.).
"Rob's a total train brain. He can be hard to deal with sometimes, and if you get into a conversation about Star Wars with him, you're never getting out."
by stwlam December 19, 2017
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