Isn't Bart entirely grade-separated? Yes, but portions run near/under/over/alongside/in the median of roadway traffic. If you read the tweet thread you can see

Isn't Bart entirely grade-separated? Yes, but portions run near/under/over/alongside/in the median of roadway traffic. If you read the tweet thread you can see that a driver on the road was ejected into the BART ROW and was hit by a train.