A failure to follow one or more safety rules designed to protect rail workers likely played a role in the Amtrak crash that killed two men Sunday, sources with knowledge of the crash said.

The National Transportation Safety Board has said the communication between dispatchers and the work crew on the track will be one of the factors examined in the investigation. On Tuesday, several sources said a possible focus was on the failure to properly secure the permission that work crews need to safely access tracks.

Amtrak Train 89 slammed into a backhoe shortly before 8 a.m. Sunday near Booth Road in Chester. The crash killed two Amtrak workers, Joseph Carter Jr., 61, of Wilmington, and Peter John Adamovich, 59, of Lincoln University, Pa., and injured 37 passengers. The train's engineer hit the brakes five seconds before impact, when the train was traveling 106 mph.

Permission to work on the tracks, called "foul time," requires specific communication between a work crew foreman and a dispatcher, rail experts said. Depending on the permission given, there is a setting, called "blocking devices applied," that electronically prevents a dispatch control center from routing trains onto a track. Alternatively, shunts are used to alter the electric current on the rails and activate signals alerting a train that a track is occupied.

The morning of the crash, an overnight crew had been working on the rails with foul time in place, sources said. Investigators are looking at the possibility that as a day crew began track work, the foul time was improperly canceled.

There are four separate tracks controlled by Amtrak in the area of the crash. The equipment, a backhoe and a ballast sweeper, was on tracks two and three. Track two was inoperable, officials said, but, because foul time was released on track three, trains were free to travel on it. The backhoe and Train 89 were both on track three.

The NTSB, which is conducting the investigation into the crash, would not comment.

The Brotherhood of Maintenance of Way Employees, a division of the Teamsters, issued a statement Monday accusing Amtrak of having "systemic deficiencies" in its safety culture. It cited inadequate reporting requirements, dismissal of veteran railroad managers, draconian discipline, and inexperienced staff as factors that were making rail work for Amtrak less safe.

The letter noted that, since March 1, there have been three track-worker fatalities on the Northeast Corridor.

Amtrak described its safety regulations as strict and noted it had a strong safety record transporting more than 30 million passengers a year on 21,000 miles of rail.

"Everyone at Amtrak is deeply saddened by the deaths of two Amtrak employees and injuries to our customers," the rail agency said in a statement Tuesday. "We are working with the NTSB to identify the issues that led to this incident, and will make any needed changes immediately."

Investigators have said Positive Train Control, an automatic braking system, was active at the crash location, and said they would be looking at the role it played, as well.

SEPTA uses a PTC system like Amtrak's; Jeff Knueppel, SEPTA's general manager, said its PTC system can be programmed with the location of rail crews and could automatically stop a train before it gets to an area where workers are on the tracks. He noted he was not certain whether Amtrak had identical features on its PTC.

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