Actually it can be fairly simple thing to rule out dirt bikes if there are no recent tire tracks at the point of origin (POI). Assuming that firefighting efforts have not destroryed the evidence at the POI is is possible read the vegetation burn patterns to determine within a couple of square feet where the fire ignited. Sometimes it is possible to find the ignition source (ie: match, incideary device, discarded smoking material, exhaust carbon from gas or diesel engines, hot metal and so on). Determining the responsible train is fairly easy by corrolating the report time with the time of the last train passing the fire location. Wildland fire investigation is an interesting science. Do a Google search sometime, I'm sure there is a lot of information on the subject.