The first two cars were packed as usual for the morning rush, so tightly that people stood pressed flesh to flesh, sandwiched between bicycles and the few seats, many without so much as a strap to hold onto.
This train didn't lurch, though. It had trouble stopping at all, overshooting platform after platform and missing at least one station entirely as it rushed toward the end of the line.
The train didn't come to a halt until it had slammed into a metal barrier at Buenos Aires' Once station. With eight cars carrying a mass of steel and humanity _ more than 1,200 people on board _ the momentum was devastating. Forty-nine people were killed and 600 were injured.
Windows exploded and the first cars were crushed into a jumble of glass, metal, plastic and bodies.
The cause wasn't immediately determined, but many pointed to a deteriorating rail system and train cars that lack modern equipment and safety measures. Passengers said the conductor had appeared to be struggling with the brakes before the crash.
The dead included 48 adults and one child _ most of whom had crowded into the first two cars to get ahead of the rush-hour crowds on arrival. The injured included 461 who were hospitalized, Transportation Secretary J.P. Schiavi said.
Passengers' friends and relatives were still rushing around the city hours later, checking emergency rooms and the city's two largest morgues for some sign of their loved ones.
Ezequiel Mercado, his mother-in-law and 10 other friends and family members frantically searched for his wife, Sabrina Espindola, 29, who didn't show up for work Wednesday. They checked nine hospitals before heading to the morgue.
"I went everywhere. She is always with her Blackberry. We are always in contact," he said. "This morgue is the last place I thought of, but, well, she's missing. I call her cell phone, and it rings, rings, but she isn't responding."
Schiavi defended the rail system at a news conference.
"It was an accident like those in many other countries," he said, pointing to a newspaper clipping about a fatal crash in Los Angeles. "In recent years, we've made huge investments" in the system.
As Schiavi spoke, riot police faced off against angry passengers in the closed station, where emergency workers spent hours extracting dozens of people trapped inside the train's first car. Rescuers had to slice open the roof and set up a pulley system to ease them out one by one. Dozens of the injured were lined up on stretchers on the station platform.
The 28-year-old conductor, who survived the crash, was apparently well-rested, Schiavi said, having just begun his workday.
"Tiredness, his age, the problems that a conductor might face" are among the factors being investigated, he said. "This young person had just begun his shift moments before the accident."
The motorman was hospitalized in intensive care and hasn't given a statement, Schiavi added.
Passengers said the conductor seemed to struggle with the brakes, missing his stopping marks at station after station.
Concepcion Ortiz, 60, told the Clarin newspaper that in Caballito, the conductor "went past the station and needed to go in reverse." Jorge Medina told the paper the same thing happened in the Haedo and Ramos Mejia stations.
"We had noticed severe problems in the brakes during the whole trip," said a man with a broken ankle who gave his name only as Juan, and was interviewed in a wheelchair at Ramos Mejia Hospital, one of a dozen that cared for the injured.
"It was noticeable because when he got to where he needed to stop, he didn't slow down. In Haedo, almost half the train passed the station; in Liniers he also overran (the platform) and in Floresta he simply went right past it. He never stopped," the young man told the C5N television channel.
A labor union official said earlier Wednesday the train had appeared to be in good working order.
"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," union chief Ruben Sobrero told Radio La Red.
Schiavi said the train was recorded slowing from about 30 miles per hour (50 kph) to 12 miles per hour about 40 yards (meters) before the impact. "We don't know what happened in those final 40 meters," he said.
The train slammed into a shock-absorbing barrier at 8:33 a.m., smashing the front of the engine and crunching the much lighter cars behind it. The second car penetrated nearly 20 feet (six meters) into the next, Schiavi said.
Most damaged was the first car, where passengers shared space with bicycles. Survivors said many people were injured in a jumble of metal and glass. Security camera images showed windows exploding as the cars crumpled into each other like an accordion, with a man on the adjacent platform scrambling across the tracks to escape the wreck.
It was Argentina's deadliest train accident since Feb. 1, 1970, when a train smashed into another at full speed in suburban Buenos Aires, killing 200.
President Cristina Fernandez canceled 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 fares are relatively low compared to other countries in the region.
Union leaders blamed what they called a history of failure to invest in maintaining or replacing aging trains.
The Trains of Buenos Aires company promotes its low fares on its website, saying that passengers pay just 23 cents a ride on average, compared to 80 cents in Santiago, Chile, and $1.11 in Sao Paulo. But TBA also complains that without higher fares, maintenance is a struggle.
Employee salaries and benefits have soared nearly 900 percent in the last decade, while the TBA now spends just 12 percent of its operating costs on maintenance, the company said.
Railway experts told The Associated Press that accidents like this are common among older trains that lack modern designs. The TBA has bought many cast-offs from other countries that have modernized their systems, and uses hundreds of Japanese-made "Toshiba Classic" rail cars that were built in the 1960s.
"That's a very slow speed" for so many casualties, said University of Southern California engineering professor Naj Meshkati, who studies rail disasters. "It's important to look at the age of the cars."
Trains using older cars can lesson the danger by running an empty and locked "deadhead" car between the engine and the passenger cars, said Colin Fulk, a rail expert and consultant in Sherrills Ford, N.C. Argentina's commuter trains don't do this.
The TBA said it is cooperating in the investigation.
"This is not an accident whose causes will be hidden from view in any way," Schiavi promised, noting that recorders, security cameras, computer systems and other evidence would be handed to investigators.
There have been a half-dozen serious train accidents in Argentina in the last 15 months. Last September, a bus driver crossed the tracks in front of an oncoming train, killing 11 people. Two months later, a bus driver transporting children on a field trip drove in front of a train, killing eight schoolgirls.
Debora Rey in Buenos Aires and Martha Mendoza in Santa Cruz, Calif., contributed to this report.
Follow Michael Warren on Twitter at http://twitter.com/mwarrenap