At least five people were killed and six critically injured on Tuesday evening when a long-distance passenger train derailed in northern Philadelphia in a crash the city's mayor described as an "absolute, disastrous mess".
The incident is the latest in a series of deadly incidents involving US trains that have raised questions about the standards and funding of systems that are ever busier but receiving little investment.
However, it also appeared from television pictures that the Amtrak train, travelling from Washington, DC, to New York, had narrowly avoided colliding with a nearby train of tank cars, which could potentially have made the incident far more serious.
"I've never seen anything like this in my life," said Michael Nutter, Philadelphia's mayor, at a briefing at the scene of the crash in the northeast of the city.
Witness accounts suggested that the train - on the busiest route for the state-owned passenger train operator - braked sharply as it entered a curve at about 9.30pm. A series of passenger cars overturned.
Mr Nutter, after confirming "at least" five deaths, said seven of the train's carriages - including the locomotive - were in "various stages of disarray - turned over, upside down, on their side".
He subsequently told CNN, despite initial reports another train might have been involved, that there was no indication of a second train's involvement.
News footage showed the train's first carriage had been reduced to little more than a mangled clump of metal, apparently as a result of hitting a pylon for the line's overhead electric supply.
Amtrak said there had been 238 passengers and five crew on board the train when it derailed. It said it was suspending services between New York and Philadelphia, a critical route for business travellers.
Patrick Murphy, a former US congressman who was on board the train, tweeted pictures of firefighters and state police trying to help passengers out of a derailed car, passengers covered in dust and the train's wreckage.
Paul Cheung, an Associated Press manager on board the train, told the wire service the train had decelerated as if someone had slammed on a brake. Everything then started to shake and passengers' belongings started to fly around, he said.
Mr Nutter said the National Transportation Safety Board, which investigates serious transport incidents, was on its way to the scene. Philadelphia's fire chief said six critically injured people had been taken to hospital. Another 44 were being treated for less serious injuries.
Tuesday's derailment follows a crash in February in Valhalla, New York, on a level crossing between a car and a commuter train operated by Metro North, one of the busiest US commuter rail operations. That incident killed five rail passengers and the car's driver. In December 2013, another Metro-North train derailed at speed on a bend in Spuyten Duyvil in northern New York City because of excessive speed, killing four people.
While Tuesday's incident bears superficial similarities to the Spuyten Duyvil crash because of its location on a bend, investigators will also look carefully for evidence of broken rails and other common causes of derailment.
While rail moves about 40 per cent of inter-city freight in the US, it accounts for only 1 per cent of passenger journeys.
Although there have been concerns about standards at a number of passenger train operators, including Metro-North and the Washington DC metro, rail safety remains far higher than on US roads, where about 33,000 people die per year.
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