( CNN ) -- A Chicago commuter train that was parked in a service yard somehow moved onto a rail line and smashed into an oncoming train early Monday , injuring dozens of people . How it happened is a mystery . `` I do n't know the last time , if ever , that this has happened '' on the Chicago Transit Authority system , spokesman Brian Steele said . There are `` more questions than answers '' about the incident in Forest Park , Illinois , he said . At least 48 people were injured ; 33 of them were transported to hospitals , Forest Park Mayor Anthony Calderone told CNN affiliate WLS . They are believed to have minor injuries . It was not immediately clear how many passengers were aboard the train . Investigators are not characterizing the incident as a runaway train at this point , Steele said . A central question : whether anyone had climbed on board the empty train and set it in motion . `` In order for a train to move , it has to be energized , '' said Ronald Ester , vice president of CTA Rail Operations . `` We call it unlocking the master controller , '' he said . The train would have needed to be placed in power position manually . Authorities are looking at video feeds from the platforms and from some rail cars . They 're also interviewing workers who were nearby . It 's unknown whether there were criminal activities , Steele said . Authorities did not immediately find windows broken , doors pried open or graffiti on the train . The train that caused the accident had four cars ; the one with passengers aboard had eight . The incident took place about 8 a.m . Train service continued but did not stop at Harlem , near the site of the crash . Shuttle buses were made available . Steele emphasized that the CTA has `` a very strong safety record . '' More on the accident from CNN affiliate WGN .
NEW : The train that caused the accident had been parked in a service yard . NEW : Authorities are investigating whether someone climbed on board . NEW : 48 people were injured ; 48 taken to hospitals , WLS affiliate CNN reported . The crash affected Illinois 's early Monday morning commute .