A little more from the above story;
Moscow’s emergencies agency press service says the train derailed because it had to brake too suddenly.
“At 8:39am Moscow time [04:39 GMT] on a stretch between stations of Park Pobedy [Victory Park] and Slavyansky Bulvar there was abrupt deceleration of a train,” a press service employee told RT.
If the driver suddenly braked, was there some obstruction on the line or something, or was the train perhaps going too fast for a curve?