The unattended freight train diesel broke loose Wednesday afternoon and rolled uncontrolled through several crossing points, striking three cars.
"Either the brakes weren't set or they failed," said Mayor Michael Bloomberg. "We don't have an answer as to why."
The New York Times reported that railroad maintenance workers repairing the tracks literally ran for their lives as the locomotive bore down on them.
The diesel engine smashed into two maintenance trucks on the tracks. One of them, a welding truck, burst into flames. The truck became wedged between the train and the tracks, helping to finally slow and stop the locomotive, the newspaper said.
Flames from exploding acetylene tanks on the welding truck engulfed the locomotive in flames. Nevertheless, a New York City firefighter, Lt. William Pickett, leaped aboard and shut the locomotive down.
The diesel had traveled more than a mile before stopping a few blocks shy of the old Bushwick terminal in Brooklyn.
The four injured were taken to Elmhurst Hospital. Jason Kusinitz, 33, of Queens, had his spleen removed and was listed in critical condition with multiple injuries, Metropolitan Transportation Authority spokesman Tom Kelly said.
Demutrius Cuffie, 38, of Queens, was in stable condition with multiple injuries. Rabbi Meyer Mallave,73, of Queens, suffered back and leg injuries. Sister Ave Clark, a nun with the Amityville Dominican Order on Long Island, had broken wrists and heels.
Officials said the train crew would be tested for drugs and alcohol as part of an investigation. The National Transportation Safety Board said it would send a team to investigate the incident.