It's your train of thought, which all goes to naught if what you end up with isn't what you sought.
"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
To Brain Train someone is to "school" them, or "shut them down" with a direct factual comment, sometimes with the help of sarcasm, (but not always) that renders them speechless.
Dude 1 Says: I think my rotor's need replaced because everytime i start my car it squeals.
Dude 2 Says: (with sarcasm) Yeah...Because that wouldn't be your belts needing replaced.
Dude 3 Says: He just totally Brain Trained you!