The National Transportation Safety Board recommended the Chicago Transit Authority take steps to prevent similar accidents, the Chicago Tribune reported.
More than 30 people were injured Monday when the ghost train rear-ended a Blue Line train stopped at a station. None of the injuries was serious.
The NTSB said trains are "routinely left powered-up while stored and with the brake setting that would allow movement.''
A source close to the investigation told the Tribune the train involved in the crash might have been sitting unoccupied and with power to the propulsion system for several days.
Investigators also said the train was stopped automatically several times by trip mechanisms.
"After each stop, the train started moving again because the master lever on the operator console had been left in a setting that allowed the train car brakes to recover and reset from the emergency brake application and proceed through a mechanical train stop mechanism after a momentary stop,'' the NTSB report said.