 The NTSB investigation found that after a June 17 replacement of a track circuit component at what became the site of the June 22 collision , the track circuit had been suffering from parasitic oscillations which left it unable to reliably report when that stretch of track was occupied by a train . The struck train came to a stop because of traffic ahead . Because the entire train was within the faulty circuit , it became invisible to the Automatic Train Control ( ATC ) system . The train behind it was therefore commanded to proceed at 55 mph . The operator of the striking train applied the emergency brake after the stopped train came into full view but there was not enough time to prevent the collision , which occurred at approximately 44 mph . 
