Federal Police Commissioner Nestor Rodriguez says the dead include 48 adults and one child.
At least 550 people were injured, and emergency workers were slowly extracting dozens of people who were trapped inside the first car, said Alberto Crescenti, the city's emergency medical director. Rescuers carved open the roof and set up a pulley system to ease them out one by one.
The commuter train came in too fast and hit a shock-absorbing barrier at the end of the platform at about 16 mph (26 kph), smashing the front of the engine and crunching the leading cars behind it; one car penetrated nearly 20 feet (six meters) into the next, Argentina's transportation secretary, J.P. Schiavi told reporters at the station.
The conductors' union chief, Omar Maturano, told Radio 10 that the train might have come in as fast as 18 mph (30 kph).
Most damaged was the first car, where passengers share space with bicycles. Survivors said many people were injured in a jumble of metal and glass. Images from a security camera show windows exploding as the first two passenger cars crumple into each other like an accordion, with a man on the adjacent platform scrambling across the tracks to escape the wreck.
The rush-hour train was 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 suffered bruises or lesser injuries, waiting for attention on the station's platforms as helicopters and dozens of ambulances carried others to nearby hospitals. About 200 people had serious injuries, said city health minister Jorge Lemus.
The dead, meanwhile, were carried out the back of the station in body bags, beyond the view of television cameras, the Clarin newspaper reported.
The toll makes it Argentina's deadliest train accident since Feb. 1, 1970, when a train smashed into another at full speed in suburban Buenos Aires, killing 200 people.
President Cristina Fernandez cancelled her day's agenda due to the accident, which raised fresh doubts about government investment in the train system millions depend on. While largely privatized, the system depends on huge state subsidies, and passengers pay relatively little compared to other countries.
There have been a half-dozen serious train accidents in Argentina in the last 15 months. Last Sept. 13, a bus driver crossed the tracks in front of an oncoming train, killing 11; two months later, another bus driver on a field trip drove in front of a train, killing eight schoolgirls.
"The series of train accidents hurts, and exposes the reality of of a state incapable of controling and acting to protect the passengers," opposition leader Ricardo Alfonsin tweeted.
Union leaders blamed what they called a history of disinvestment in maintaining or replacing aging trains.
"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, train workers' 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.