Rail has upper capacity in the ranges of 60-90k passengers per hour, whereas car lanes are in the range of 2-3K passengers per hour in free flowing conditions. Granted that 60-90K figure doesn't involve people trying to shove furniture, pets etc. into their vehicles, but you could move quite a lot more with rail.
Rail is also used as part of the evacuation, but most people will want their car so they can carry enough supplies to last for the evacuation period. In fact one of the most egregious aspects of the Katrina disaster was New Orleans explicitly told Amtrak their trains were unnecessary [1].
>"We offered the city the opportunity to take evacuees out of harm's way," said Amtrak spokesman Cliff Black. "The city declined."
