The engineer at the controls refused to give a statement to authorities and left a police station with a lawyer, police said.
More than 200 people were injured in the derailment which plunged screaming passengers into darkness and chaos. It was America’s deadliest train accident in nearly seven years.
Hours after recovering the locomotive’s data recorder, the National Transportation Safety Board tweeted that the train “exceeded 100mph” before jumping the tracks in an old industrial neighbourhood not far from the Delaware River shortly after 9pm local time on Tuesday.
The finding appeared to corroborate an Associated Press analysis of surveillance video from a spot along the tracks. AP concluded from the footage that the train was speeding at approximately 107mph (170kph) just before it entered the bend.
The speed limit is 70mph (110kph) just before the bend, the Federal Railroad Administration said.
The engineer’s name was not immediately released.
Technology that could have prevented yesterday's Amtrak crash was supposed to get installed in Philadelphia this year http://t.co/3FXq6Jms1e— Mashable (@mashable) May 13, 2015
The accident closed the nation’s busiest rail corridor between New York and Washington – causing chaos to the morning commute and forcing thousands of travellers to find some other way to reach their destination – as investigators examined the wreckage and the tracks and gathered other evidence.
The dead included an AP employee and a midshipman at the US Naval Academy.
Philadelphia Mayor Michael Nutter said some people remained unaccounted for, though he cautioned that some passengers listed on the Amtrak manifest might not have boarded the train, while others might not have checked in with authorities.
The train was en route from Washington to New York with 238 passengers and five crew members listed on board. The notoriously tight bend is not far from the scene of one of the nation’s deadliest train crashes more than 70 years ago.
Passengers scrambled through the windows of toppled carriages to escape. One of the seven coaches was severely mangled. Hospitals treated more than 200 people for injuries that included burns and broken bones. At least 10 remained in hospital in a critical condition.
Amtrak inspected the stretch of track, just hours before the accident, and found no defects, according to the Federal Railroad Administration. In addition to the data recorder, the train had a video camera in its front end which could yield clues to what happened, said NTSB member Robert Sumwalt.
The area where the crash happened is known as Frankford Junction, situated in a neighbourhood of warehouses, industrial buildings and homes.
It is not far from the site of the 1943 derailment of the Congressional Limited, from Washington to New York, which killed 79 people.
Amtrak carries 11.6 million passengers a year along its busy Northeast Corridor, which runs between Washington and Boston.
The mayor, citing the mangled tracks and downed wires, said: “There’s no circumstance under which there would be any Amtrak service this week through Philadelphia.”