(CNN) –

Amtrak was spending the weekend installing new speed controls on the section of track where one of its passenger trains derailed near Philadelphia, as investigators examined reports that the New York-bound train was one of three hit by an unknown object shortly before the fatal crash.

The installation of the Automatic Train Control (ATC) system to slow speeding trains follows an order by the Federal Railroad Administration to Amtrak to take immediate steps to improve safety on the busy Northeast Corridor.

The railroad administration also instructed Amtrak to assess the risk of all curves on the corridor where the approach speed is significantly higher than curve speed, and to increase speed limit signage throughout the corridor.

Amtrak said it would immediately implement all three measures. The acting administrator of the FRA, Sarah Feinberg, said Amtrak wants to resume Northeast Corridor service at full capacity by Monday or Tuesday, and that these measures must be taken first.

Speed controls at the crash site

The Amtrak train that crashed was traveling from Washington to New York when it derailed Tuesday night on the curve at Frankford Junction, just north of Philadelphia. Eight people were killed and more than 200 were injured. Seventeen people remained in the hospital Saturday.

ATC speed controls are in place at the curve for southbound trains, which enter the 50-mph curve from a maximum speed of 110 mph, Amtrak says. But they are not in place for northbound trains, which enter from a maximum speed of 80 mph.

“The rationale behind the decision not to install there (which was made in the ’90s) is that the drop in speed (from 80 to 50) is considered within the risk envelope,” Amtrak explained to CNN via email. “Going the other way, the decrease in speed is much greater going into the curve (110 to 50) so that’s why ATC was installed there.”

The train that crashed this week, Northeast Regional Train 188, was traveling at 106 mph when it derailed, the NTSB has said.

“Had the train been operating at max authorized speed heading into the curve, it would not have come off the tracks,” Amtrak wrote.

Reports of objects hitting trains

Meanwhile, the National Transportation Safety Board and FBI was investigating reports that the Amtrak locomotive and two other trains in the vicinity were hit by objects just before the crash.

The Amtrak locomotive’s windshield was shattered in the derailment, but NTSB member Robert Sumwalt said the FBI is examining particular damage to the lower portion.

The assistant conductor on the Amtrak train told investigators she overheard engineer Brandon Bostian say in a radio transmission that the train had been struck by something, according to Sumwalt. Investigators have interviewed Bostian, who suffered a concussion and other injuries, but Sumwalt said he has no recollection of events just before the derailment.

A regional train passing through the same area also suffered windshield damage from an unknown object and had to make an emergency stop.

Southeastern Pennsylvania Transportation Authority police responding to the incident said in radio transmissions, “It is a train that had a foreign or unknown object make contact, shattering the windshield.”

The Amtrak train’s assistant conductor said she overheard the SEPTA train’s engineer say the windshield was either hit by a rock or shot at.

A passenger on the SEPTA train told CNN he heard a loud boom before the train came to a stop, and the engineer, who appeared shaken, told passengers something had hit the train.

A photo of the front of the SEPTA shows a circular crack on the windshield.

Adding to the mystery are reports from passengers aboard an Amtrak Acela train from New York to Philadelphia that something hit and cracked a window roughly 20 minutes before the Amtrak crash.

All three incidents happened near the 30th Street station in Philadelphia.

Investigators are considering all possibilities, including speed, track conditions and human error, as they try to figure out what happened.