The source was likely some red hot wheels and brakes. One of the eyewitnesses leaving the MusiCafe remarked about some of the wheels being red hot -- probably the ones whose manual brakes had been set and were dragged all the way down the hill.
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?