On February 3, 2004, at 5:46 p.m., northbound Chicago Transit Authority (CTA) Purple Line train 509 collided with the rear car of standing Brown Line train 419. The collision occurred just north of the Merchandise Mart passenger platform (MART) during the evening rush hour in Chicago, Illinois. The trains were operating in Automatic Train Control (ATC) cab signal territory.2 The collision occurred on track that is elevated about 20 feet from street level on a series of open deck bridges. No cars derailed as a result of the collision, but 42 passengers sustained minor injuries. A third train (Brown Line train 422) neared the striking train but stopped short of a second rear-end collision. Weather conditions were clear and dark; the temperature was 18° Fahrenheit.