Why do they make subway cars so tough?

Living in nyc now I've been riding on the subway quite a bit.  I've noticed that the subway cars are really strong and heavy, made from lots of steel.  Why do they need to be so strong? I feel like they could have a thin walled skiff and that would mean enegy savings not to mention improved schedules since they could stop faster.  Now, they just seem way ovetengineered.

-- Sent from my Palm Pre