The freeways will be contraflow. But the local roads (2 to the northeast, 1 to the north, 2 to the northwest, and 2 or 3 west of the Mississippi River) AFAIK will still be 2 directional, allowing return trip for evacuee buses(if they run any.) If they had a good plan in place (and after watching the scared mommy governor I am starting to have doubts about that) they could set up inland shelters and make several runs using buses. Might have had to take on the unions to get that implemented, might have had to take on inland local gov't's who didn't want the problem that temporarily sheltering 'those types' would bring (which if the worst happened could turn into a long-term problem), so politics could easily have prevented a good plan from being implemented, but it certainly was possible. Not to mention that Amtrak has a major base of operation in NO, with spare equipment, that could have been planned to commandeer and run shuttle evacuee trains to shelters in Baton Rouge, Lafayette, or Alexandria. Somewhat logistically complicated, but again certainly possible to avoid the worst case, and the gov't has the power to order the railroads to clear the tracks, allowing such trains to shuttle back and forth at 60mph most of the way. Too late to organize on the fly, but they had lots of options if they had chosen to plan for them. We'll see.
http://www.lsp.org/lcadeg.html
This is a useful, but somewhat confusing site.
Good thought. Empty freight cars can carry loads of people. Actually, my only time in NO was passing through on a train, but it was a passenger route, not the hobo variety. Prayers especially for the poor, sick, and disadvantaged.