I’m guessing that the bus got high-centered crossing the track. This eyewitness says the bus was stuck fo 5-10 minutes before train hit it. It seems like it would have be a good idea to evacuate the bus once they got stuck on a railroad crossing! http://www.sunherald.com/news/local/traffic/article136966968.html
Another eyewitness featured on WLOX-TV news contradicts that eyewitness. She said the bus was stuck on the tracks only a short time--less than a couple of minutes--before the train came.
Offloading a bus full of elderly folks cannot by accomplished quickly--even when a big freight train is coming.