The federal government has ordered Amtrak to install automatic braking systems on the stretch of track where a train moving at twice the speed limit derailed at a curve in Philadelphia, killing eight people.
The Federal Railroad Administration also ordered Amtrak on Saturday to assess all lines in its busy Northeast corridor to find curves where trains must significantly slow down.
"These are just initial steps, but we believe they will immediately improve safety for passengers on the Northeast corridor," said Sarah Feinberg, acting federal railroad administrator.
Speed is at the center of still-unanswered questions about the derailment: Why did Amtrak 188 accelerate Tuesday night to more than 100 mph in the last minute before it derailed on a curve with a 50-mph speed limit?
The railroad administration demanded that Amtrak install the safety braking system on its northbound railways near the derailment site and on all similarly curved railways. The system, called automatic train control, can apply a train's brakes if an engineer fails to slow it down below speed limits.
Automatic train control is a basic version of the more complex positive train control, which uses global positioning satellites, sensors and sophisticated computers to monitor trains and automatically prevent crew members from ignoring signals and speed limits.
Federal investigators have said that positive train control would have prevented the derailment. Amtrak has installed the system on parts of the corridor, but the northbound stretch where the derailment occurred had not yet been equipped.
"We are continuing to work with the [National Transportation Safety Board] to understand exactly what happened on Tuesday so we can prevent this type of devastating accident from ever happening again," said U.S. Transportation Secretary Anthony Foxx.
Amtrak must install the automatic train control system before it can resume service in the area of the derailment site, called Frankford Junction, in northeast Philadelphia.
"We will immediately implement the Federal Railroad Administration's directives to further improve passenger train safety along the Northeast Corridor," Amtrak said in a statement Saturday.
The mystery of the acceleration in Tuesday's derailment only deepened when the train's engineer, Brandon Bostian, 32, told investigators he could not recall anything after ringing the train's bell as it left the Philadelphia station bound for New York.
NTSB member Robert Sumwalt said Bostian was cooperative in his Friday interview, but had no recollection of any discussion with another engineer or of the derailment, which injured more than 200 people in addition to those killed.
Investigators are trying to determine the significance of circular windshield damage to the train, and the NTSB has asked the FBI to determine whether the train was hit by an object before it derailed. One of the Amtrak train's conductors told investigators she thought she overheard a radio conversation between Bostian and another engineer on a regional commuter train. The other engineer said the train had been hit by a rock or shot at, Sumwalt said. The unidentified assistant conductor told investigators that she thought she heard Bostian say their train had also been struck.
The idea of an object hitting the train first came up the night of the crash. Minutes before Amtrak 188 derailed at 9:21 p.m., two other trains had windows broken, possibly by thrown objects, in the same area and around the same time of the crash, according to passengers interviewed by the Los Angeles Times.