At least 90 people were injured in Argentina's capital when a passenger train struck an empty boxcar on the tracks and derailed Friday, authorities said, a rare collision that fueled questions about basic safety.

The train was on its way from Buenos Aires to the northern suburbs when it derailed around 10:30 a.m. on a bridge in the trendy neighborhood of Palermo, safety officials said. Authorities said it was not immediately clear why the empty boxcar had been on the rails but that they were investigating.

"There is not enough information about the mechanics of this accident," Buenos Aires Mayor Jorge Macri said from the crash site where he praised the swift evacuation of victims.

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Dozens of injured were treated at the scene and 30 people taken to hospitals in moderate to serious condition, at least two by helicopter with chest trauma and broken bones. Alberto Crescenti, director of the city's emergency service, said rescuers with police dogs had helped 90 people trapped in the train's wreckage, lowering some from the highway overpass by rope.

Dazed passengers staggering out of the derailed boxcars told local media the train had stopped on the bridge for several minutes before starting up again and slamming violently into the other train, jolting passengers and veering off the rails in a jumble of sparks and smoke.

Officials at the Argentine rail authority, Trenes Argentinos, said service on the popular rail line had been suspended as was underway to secure the area. They asked residents to stay away from the crash site to make room for emergency responders.

The collision brought increased scrutiny to rail safety in Argentina, where a string of several train crashes from 2012-2014 left over 50 people dead and hundreds injured.

It emerged at the time that outdated infrastructure, delays and human error had left the railway system vulnerable to crashes, prompting the government to invest in new safety and braking systems.