During the morning rush hour on January 9, 1976, two trains collided at Addison on the Kennedy Extension, in what was the most serious "L" accident up to that time (though unfortunately surpassed just 13 months later).

The National Transportation Safety Board (NTSB) summarized the accident and the probable cause thusly:

On January 9, 1976, at 8:06 a.m., Chicago Transit Authority (CTA) train No. 315 struck the rear end of train No. 104 while it was standing at the Addison Street Station platform in Chicago, Illinois. The impact forces extensively damaged the lead car of the moving train and the rear car of the standing train, and slightly damaged the other cars in both trains. Damage to the equipment and track was estimated to be $267,000. Of the 381 passengers who were injured in the collision, 1 passenger died.

A southbound 'B' train of 2200-series cars, Run 315, struck the rear of an 'A' train of 6000-series cars, Run 104, standing in the Addison station -- at the time, the Addison station was an 'A' station, and so Run 315 would have normally passed through without stopping. The cab signal (automatic train control, or ATC) system was having issues (which had been common on the Kennedy Extension at the time), and Run 315 had bypassed the cab signal system to allow the train to proceed down the line. In addition, some visibility factors existed: while the Addison station is reasonably visible approaching from the previous station (Irving Park) due to the presence of a down-slope between the two stations, just before a train gets to Addison station from Irving Park there is a significant curve. It was also a bright, sunny morning, and glare may have been a factor.

The NTSB determined the probable cause:

The National Transportation Safety Board determines that the probable cause of this accident was the failure of the motorman of train No. 315 to perceive standing train No. 104 at a sufficient distance to permit him to stop his train before striking No. 104. Contributing to the collision were the rule that permitted the operation of the train with the automatic train control and the cab signals inoperative, the lack of consistent enforcement of operating rules, the absence of flag protection against following trains, the failure of the train phone system to provide reliable communications, and the violation of the 25-mph speed limit required by Rule 178B.

Car 2308, the lead car of the striking train, was seriously damaged and retired, its mate renumbered a paired with another car.

NTSB Accident Report Detail: Reports, recommendations and other documents from the National Transportation Safety Board about the accident.

WLS Channel 7 - Special Report: "CTA Train Collision": YouTube video

WBBM Channel 2 news teaser and photos: YouTube video