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Fatigue or temporary inattention have been raised as possible factors in other commuter train accidents.
In December's train derailment that killed four people in New York, representatives of the operating engineer have said he may have lost focus at the controls in a momentary daze. A preliminary report did not mention that issue, saying excessive speed appeared to be a factor.
That accident highlighted the lack of crash-avoidance systems, or "positive train control," which uses GPS, wireless radio and computers to monitor a train's position. It lets dispatchers halt engines remotely if they speed or blow through stop signals.
It's not clear such a pricey system could have helped prevent Monday's derailment in Chicago.
"There are systems that do stop trains," DePaepe said. "But it is usually about money. The transit agencies do the best they can."
Monday's accident occurred almost six months after an unoccupied Blue Line train rumbled down a track for nearly a mile and struck another train head-on at the other end of the line in September. Dozens were hurt in that incident, which prompted the CTA to make several safety changes.
While the station is closed, the CTA will bus passengers to and from O'Hare to the next station on the line.