What is the purpose of the lower seat back on the aisle seat of some commuter trains?

Question

I just saw it in the movie "The Happening" and it made me remember when I took a commuter train into NYC and wondered the same thing. You know, there are three seats on one side, but the window seat and middle seat have higher seat backs than the aisle one. I've looked online and can find no explanation.

Answer 1

It makes for easier exiting. In case of emergencies, you don't have to climb over a high backed seat to get out.

Answer 2

They save money with the low seat back.