Investigators believe no one was on the train when it traveled about three-quarters of a mile and hit another train, injuring 33 people, the Chicago Sun-Times reported Wednesday.
"From all indications of a very thorough and comprehensive examination at the crash site by our investigators, they certainly are of the opinion that no human was on there,'' said Forest Park Mayor Anthony Calderone said.
He added investigators believe "some type of mechanical malfunction" caused the Monday morning rush hour collision.
Forest Park police examined transit system videos from several cameras along the train's path, Calderone said. They found no evidence someone got on the train, started it and then jumped from the train while it was in motion.
The train was out of service and undergoing maintenance at the Chicago Transit Authority service yard when somehow it started, slipped past two switches and an internal device that should have stopped it before it hit a train traveling in the opposite direction on the same tracks, said Robert Kelly, president of Amalgamated Transit Union Local 308.
The other train was carrying 40 passengers. None of the injuries were considered serious.
Kelly added that switches on the track should have stopped the unmanned train as it traveled the wrong way.