DC Metro Trains Collide, Many Injuries

Two DC Metro trains collided at about 5pm today. The trains were above ground at the time of the collision. There are numerous injuries and at least one fatality. NBC reports:

Two Metro trains collided between the Takoma and Fort Totten stations on the Red Line, resulting in at least one fatality, D.C. Fire and EMS Department spokesman Alan Etter said. A six-car train headed toward Shady Grove derailed between those Metrorail stations and was involved in a collision with another train at 5 p.m. today, Monday, June 22, according to preliminary reports. The collision took place above ground. Officials have confirmed one fatality. There are numerous other injuries. Passengers are currently still trapped in the trains; emergency workers are using tools to cut them from the wreckage. A witness told Bensen that it appeared that one train car looked like it was on top of another train car.

Why don't these train systems have automatic shutoffs when trains get too close to each other?