It would be interesting to know what the cause was. If it was indeed the radio batteries, then my speculation would be that they were Lithium Ion, and those can cause a very hot fire. This is a problem with EVs and I believe Chevrolet Bolts were just recalled because several had battery fires.
Alkaline, NiCad or NiMH (nickel metal hydride) aren't usually associated with combustion unless it is caused by something else powered by them.
Something to think about.
When I was still working there the battery was a car battery in the fireman's seatbox with a hard wired trickle charger attached to it. The cord plugged into a droplight\extension cord that would be connected to the house power overnight.
It could have been any number of things linked to that system that went. A pinched wire, a bad charger, a blown battery. At some point I'm sure we'll all hear through the grapevine but at least it was caught with the only damage being to the cab fixtures and (what was left of) the woodwork.
I never liked the idea of plugging into shore power overnight. Aside from trusting the guy before you to plug in the charger, chargers in general just give me the heeby jeebies. I much prefer the way it's setup at Essex where the battery charger is run off the power from the generator. That limits the battery charging to when the loco is in service, which is plenty of time, anyways.