I was on a coal train on BNSF in late 1999- early 2000. My run was Creston, IA, to Galesburg,IL. Approaching Burlington, IA, a westbound reported a car of coal on fire. The fire department came to put it out.
At the time, the farmers were burning brush from their fields. Who knows if hot embers did it, or spontaneous combination. I tend to think it was spontaneous combustion, since the firefighters were using a thermal imaging tool. They were digging down because some of the hot spots were below the surface. The movement of the car certainly created enough draft, which enabled the fire to grow.