I guess FEMA may not have legal authority to force the railroads to allow it to run special trains in a disaster but it would be a real PR disaster for a freight railroad to turn down a request to evacuate people from an area where they could die or be seriously injured if they remained at the scene. If roads and airports were closed and a train was the only way out it's possible that a state of emergency could be declared that would put a lot of pressure on railroad management to cooperate. Since there aren't many spare passenger cars around there may only be enough for one or two trains anyway and if it's a really bad situation the railroads might be likely to suspend trains heading to the area so that would make room for a few special passenger trains going away from the disaster zone.