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Chicago Transit Authority are investigating how two trains — one of them out-of-service — were involved in a head-on collision Monday morning that sent dozens of people to area hospitals. The Chicago Sun-Times reports that two CTA trains collided around 8 a.m. Monday, injuring at least 48 people, though none of them appear to be seriously wounded. CTA officials disputed those numbers with the Chicago Tribune, saying only 33 people were injured. 

Officials are still trying to answer the bigger question: Why wasan out-of-service train was travelling east on the same track as a westbound train filled with commuters during the busy morning hour? Oddly, Forest Park Mayor Anthony Calderone told reporters that "It’s our understanding that there was no one on the eastbound train," and it was traveling at a "pretty low speed," according to a CTA spokesperson.

There is speculation — although the rumors are so far unconfirmed — that the out-of-service train may have been stolen or hijacked. Chicago's CBS 2 and NBC Chicago are both reporting that police are treating the crash site as a crime scene and refuse to rule out the possibility the train was stolen. (Update, 12:26 p.m. CBS News now reports, per "federal officials," that all signs point towards the crash being an accident.)

We'll have more updates as they become available. 

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