The full report may take longer, but their first report today has said that this was an “electrical arcing event” from some power cables running alongside of the tracks. There was also reportedly water on or alongside of the tracks. So it probably was just poor maintenance.
Over time, heavy current power cables heat up and can degrade. Even those packed in conduit, many cables are often worse and there are current degrading requirements based on # cables and diameter of the conduit. Rats love the covering insulation where they can get at it, too.