Oh yeah, that happens. Have to play it by ear.

In one kind of scenario, the freeing up of the seat has to be more discreet/subtle, and if there's a likely sense you did it intentionally, then an unspoken tone of "hey, I'm getting off the train, and this seat is free" or "I got your back, buddy", and maybe a hint of deferential respect if they're significant older than you (but not like you're calling them "old").