It's possible that one train was looping at Park and something happened, as it's pretty tight turn and would certainly probably not allow a car from Gov't Center to pass by along the wall track. It's hard to tell exactly what happened, but if the TP switch snapped back before the train was clear, perhaps the operator saw something and stopped the train before it was truly "derailed" but off enough to mess up the passage of trains from NS to Arlington.
It sounds to me like a defect in the switch, either mechanical or electrical. Knowing the media, the reports will say "the Green Line train that derailed tested positive..."
Be interesting to keep an eye out and see what comes of the investigation. I can't imagine there's any way a train operating the loop at Park would be speeding.