The incident site is very specific on the fire origin site, right next to the D&S tracks, and the fire origin time, Friday June 01st, 2018 approx. 10:02 AM.
It lists the cause as unknown.
Neighbors in the area spotted the fire right away and tried to put it out as did the D&S fire patrol behind the train, but the conditions were set for the fire to spread too quickly for their efforts.
Thus far the railroad has not been identified as the cause of the fire, but it is likely a suspect. Both the D&S and C&TS make fire prevention a high priority.
The cause might have been an ember from the locomotive, it might have been from something dropped or flicked by one of the passengers, it might have been as spark from the breaks on a car, it might be a hiker walking along the tracks, or any number of less likely but not impossible causes. It may have even been deliberately set by someone wanting to cause trouble for the railroad.
I will leave the determination of the cause to the experts who will investigate the cause. Until they determine the cause it is "unknown"