"There are people still trapped, people alive, and there may have been fatalities. We don't know if there are dead people" in the wreckage, J.P. Schiavi told reporters at the station.
The commuter train came in too fast and hit the barrier at the end of the platform at about 12 mph (20 kph), smashing the front of the engine and crunching the leading cars behind it, Schiavi said. One car penetrated nearly 20 feet (six meters) into the next, he said.
Most damaged was the first car, where passengers make space for bicycles. Survivors told the TeleNoticias channel that many people were injured in a jumble of metal and glass.
Passengers said windows exploded as the tops of train cars separated from their floors. The trains are usually packed with people standing between the seats, and many were thrown into each other and to the floor by the force of the hard stop.
Many people suffered bruises, and many with lesser injuries were waiting for attention on the Once (OHN-say) station's platforms as helicopters and more than a dozen ambulances took the most seriously injured to nearby hospitals.
"This machine left the shop yesterday and the brakes worked well. From what we know, it braked without problems at previous stations. At this point I don't want to speculate about the causes," Ruben Sobrero, union chief on the Sarmiento line, told Radio La Red.
The motorman has been hospitalized and the union hasn't been able to speak with him yet, Sobrero added.