I've been in enough fights on schoolbuses to know that the disadvantage of unarmed combat in close-quarters works both ways - if you can get within arm's reach of your opponent, he has very little room to evade your punches, kicks and grabs. He's soon cornered quickly. And you'd be surprised how fast a motivated person can hurdle several seats - forget using the aisle.
(That was my past - I've been walking the "peaceful path" - for now.)
Come at 'em from behind, drag them over the seat backward with a sideways twist to put them on the floor between your seat and the one they were in. (headlock preferred)
Wedge them sideways halfway between two seats if you have that style of seating, and never, never quit swinging.
I've been in a few of those dust-ups! Even better if you get them on the floor in the aisle, you can use the seats on either side to support your upper body and do damage with your feet.
To think, I spent over 100,000 miles of my life riding those d@mned things. Not every day was peaceful, by any means.