(CBS) – The Chicago Transit Authority offered little new information Thursday, a day after a Green Line derailed on the South Side.

Sources close to the investigation tell CBS 2 a non-union CTA manager was at the switch site and threw the switch the wrong way, which derailed the train Wednesday.

The third car of a four-car CTA Green Line train derailed on the elevated tracks at 63rd and Prairie.  No injuries resulted.

CBS 2 asked the CTA whether a manager was detailed to the site whether that was because of a manpower shortage.

“At this time the investigation into yesterday’s minor derailment is ongoing. Therefore we are unable to provide details about the incident or cause. This was a minor derailment and there were no injuries, however, the CTA takes every incident seriously and we are thoroughly investigating,” the agency responded in a written statement.

A spokesperson says Wednesday’s derailment was the only one of its kind in 2015, adding the CTA provides more than 2,200 train trips each week day.

“We’re glad no one was injured. We applaud everyone for their efforts for getting everyone off the train to safety. We have no comment at this time, pending the investigation,” says ATA Local 38 President Kenneth Franklin, whose organization represents motormen.