It is still not clear what caused the deadly crash of two Red Line trains in Washington, D.C., yesterday, but the National Transportation Safety board has begun an investigation into why one speeding train slammed with such force into a halted train that one actually slid on top of the other. Media reports have said up to nine passengers were killed in the accident (though the Washington Post reports it was only seven), including Jeanice McMillan, the driver of the rear train. At least 70 other people were injured, and rescue crews are using heavy equipment to try to rescue the ones still trapped inside. This was the deadliest crash in the subway system’s history, though it has experienced fatal incidents before. In this case, the train should have been operating on an automatic system. Officials don’t know why that system apparently failed, or why McMillan did not apply brakes before hitting the stopped train.