Two subway trains collided in Washington D.C. yesterday, killing seven people. It's being called the worst metro accident in the city's history. The details currently known are that during rush hour on the red line, one of the system's busiest lines, one train ran into the back of a stopped train. The National Transportation Safety Board warned city officials that the type of train involved in the accident needed to be retrofitted for safety, but the city was unable to follow through with the recommendation. Ian Urbina, a reporter for our partner The New York Times, is following the story.