R Ruiz has good points regarding the long term. But in the short term, railroads are not innocent of starting fires. Several ignition sources occur, such as hot material and sparks from overheated brakes, and occasional hot stuff from exhausts (though that's less common now that most locomotives have emission controls. That said, it's still less common for modern railroads to start wildfires than it is for highway traffic to: there are mechanical failures (Carr Fire near Redding apparently was started because somebody got a flat tire and kept driving until the tire came off and the wheel on pavement made sparks), wrecks (several grass & brush fires in recent weeks started by a car fire, and parking a car with a hot catalyst over grass will easily start fires too), and simple stupidity (arsonists, either intentional or unintentional).

Another consideration is that motor vehicles allow people to get to and live farther out in the bush. Where people are, fires start, and even if naturally started (usually lightning) having people in the way causes a need for fire suppression. A naturally-caused wildland fire in a wilderness area will inconvenience a few people who have to terminate their visit prematurely, but it's not a disaster and at most has to be watched to make sure response is called for when or if it gets into a populated area. Much like with earthquakes: the shaking doesn't kill people; the building falling down due to inadequate construction does.

It's too late now, but if railroads had continued to be the primary means of interurban transportation (rather than American Autobahns) we would likely be having less of both climate change and wildfire problems.