No chance of a DAI from bouncing in the metro.

A diffuse axonal injury results from violent impacts to the head where the brain matter violently impacts the skull. Often, it is the result of a violent twist of the head. These would be more common from car crashes where the head violently twist as it hits the head rest and door frame.

Bouncing in the metro would at most just aggravate a tender neck. Unless the metro impacted another solid object and you were flung into a seat or wall, there would not be enough energy in the movement.

Stop looking for things to worry about.