We are still a long way from know all of the details about this accident, but thanks to some detailed reporting from a local TV station and a local paper there is now an abundance of information including photos of the burning rail car.
It is now apparent that there is a single rail car involved and it is still on the tracks (though CSX is still technically calling it a derailment). The preliminary cause of the incident appears to be that a broken train axle punctured the acrylonitrile tank and produced sparks that started the leaking liquid on fire. See the photo here of where the accident actually happened.
The train car continues to leak feeding the small pool of flames (see photo here). The local fire department (almost certainly in consultation with CSX) have decided to let the fire burn until the leak stops. This will reduce the potential environmental effects from the acrylonitrile spill. Local authorities are already advising against drinking from local wells until the problem is resolved.
FRA inspectors are already on site, but will not be able to really start their investigation until the fire burns out.
I made the comment in my earlier post about the combustion byproducts for acrylonitrile. Those comments assumed complete combustion in a hot fire from a major spill. In the small scale fire like the one we are seeing in this case the combustion is not necessarily complete and there may be significant amounts of cyanide produced. News reports indicate that some of the first responders in the hospital may be suffering from cyanide exposure.