I am going to go out on a limb and suggest that wood and coal fired steam would be the most likely to start a fire due to thrown sparks/cinders, but not sure which would be worst. I can attest that wood fired engines do. Second I would place 1st generation diesels 3rd., and probably in 4th place in terms of most likely to start a row fire would be oil fired steam (not considering tie fires due to burning oil dripping which usually doesn't happen. Not sure where present day diesels would fall.
In a book I read about early dieselization on the CPR's Kettle Vly line it states that crewmen reported consideably more ROW fires with first generation diesel power than from the coal fired steam they were replacing. Also a nearby shortline operator starts one or two ROW fires a year in the grass along the track with what I guess would be called 2nd generation diesels. As where they usually start a fire is in a valley with virtually no grade I don't think anyone would suggest it is due to brakes but is rather from burning particles from the stack. To carry this a little farther, last winter someone asked me if the same shortline operator had gotten a steam locomotive as they noticed the engine in the dark throwing fire and sparks out the exhaust.
Yet another story is told on a certain class 1 that had an engine spewing hot carbon out the exhaust over a span of many miles and set quite a number of fires. The story goes that by the time the authorities got organized to investigate the cause, the engine in question was no longer an engine but a large pile of disassembled parts in a shop, thus ending further inquiry.
As Jerry said, a certain wood fired enigne we are familiar with throws a few sparks once in a while. As fireman, I have experienced smouldering grease rags on top of the fireman's seat box from a hot one that came in the front window, not to mention holes burned in my plastic lunch sack left in the same location. Further while doing a student engineer run, I had the experience of looking down to see a quarter size hole in the crotch of my overalls growing larger with a bit of orange glow around the perimiter. From the actions taken, someone standing trackside might have thought I was a bit eccentric. It is no wonder most engines that were wood fired on the SVRy or logging lines had some sort of fire hose arrangement, usually off of the engineers injector. Not sure if this was prototypical on coal fired lines during high fire danger.
