The eyewitness also described sparks (from wheel flanges grinding on the rails?). Could have also come from sparks associated with later cars still crashing into each other. Smokers out on the porch?
Probably not smokers, likely some of the few (but not enough) hand brakes set.
I’ve tried to set light crude oil on fire with a lit cigarette. (semi-controlled experiment in a safety discussion during oil field construction) I’m convinced it is essential impossible. Maybe some with a extreme amounts of specific light ends but I doubt it.