No the train is always closed during Fireworks. If fireworks are at park closing the last train will leave Main Street one hour before fireworks and do a last loop of the park so that the trains can be completely off the track and in the roundhouse before the fireworks begin. If they are running 2 parades and fireworks (with the fireworks in the middle) and the park will remain open after the fireworks the last train leaves Main Street about 30 minutes prior to fireworks, does one loop and holds at Main Street (with the other train holding at Toon Town...there are always at least 2 trains running). They will start running again shortly after the fireworks.
The back area between Frontierland and Toon Town is actually where the Main fireworks bunker is (for the BIG fireworks that look like the are above the castle) so the trains tracks back there are the fall out area and quite frequently the trees and brush back there gets set on fire (we have a fire station right there). The train service is stopped for this reason and can not resume until the fire marshall inspects the area and calls it clear.
Train service is not offered during any of the ticketed events. The Halloween party, Christmas Party, Night of Joy, e-rides, etc.