A derailment of the same routing at the same location (Riverside outbound, Kenmore to Fenway, in the Beacon Junction) happened a few years ago.
Today, one set of pictures seems to show the first truck going straight (to Beacon St) apparently because the train was going too fast for the switch to flip to the Riverside branch on the left. The switch flipped in time for the middle truck to take the turn, splitting the switch, and then the middle of the train was dragged off the track to the right to follow the front half and to hit the divider between the inbound and outbound Beacon St. tracks in the junction. The train came to a stop with the rear also making the left turn.
The last time it happened the switch must have already been/become set for Fenway/Riverside since the train began the left turn and a moment later it went off the track due to centrifugal force from excessive speed.
Many decades ago there was a crash in NYC where a switch mysteriously flipped under the 9'th car of a train and that car was dragged across an upcoming divider between the two diverging subway tracks. When all was said and done one side of that car was crushed to be about 2 feet from the other side in places, with much loss of life and limb.
Quick! Stop your bank account autopayments so you can decide where you want your unemployment check or stimulus payment to go first. (You might choose to set credit cards to minimum payment).