Federal agencies have repeatedly advised railroads to use backup safety precautions for workers on the rails after accidents hauntingly similar to Sunday's fatal Amtrak crash in Chester.

Sources with knowledge of the crash that killed two have said a communications lapse during a shift change contributed to workers' staying on the rails while safety precautions designed to route trains away from them were canceled.

This exact scenario, according to a 2014 Federal Railroad Administration safety advisory, has been an ongoing problem in railroad work.

"FRA is concerned about the infrequent, but repetitive incidents involving roadway workers being struck or nearly struck by trains that appear to be due to miscommunication," the advisory stated.

The FRA advisory stated backups were needed as a stopgap until positive train control, an automatic braking system, was installed.

Amtrak has PTC on most of its Northeast Corridor, including the area in Chester where the two workers were killed, and experts say the system hasn't eliminated the need for additional precautions recommended almost two years ago.

National Transportation Safety Board investigators have said Amtrak Train 89, which left New York for Savannah, Ga., on Sunday, was traveling 106 mph and didn't start slowing down until five seconds before striking a backhoe on the tracks near Booth Road. The train's brakes were applied by the engineer at the last moment, officials said.

The crash resulted in the deaths of Joseph Carter Jr., 61, of Wilmington, and Peter John Adamovich, 59, of Lincoln University, Chester County, and left 37 people injured.

When safety rules fail, said Allan Zarembski, a University of Delaware rail expert, there are two solutions.

"You change your procedures," he said, "or you introduce new technology to prevent this."

The 2014 FRA advisory cited three examples from 2007 to 2013 of rail worker deaths caused by communications problems. From 2010 to 2015, 92 rail workers were killed on duty, according to FRA data. Three of those were Amtrak employees.

Rail workers rely on strictly structured communication between a site foreman and dispatcher to keep trains off tracks where people are present.

The 2014 FRA advisory recommended three types of backup for cases in which there is a communications breakdown. One is a device worn by workers that would alert them to a coming train while simultaneously warning train operators of people ahead.

Another would offer similar protections for maintenance vehicles.

A third backup goes a step beyond the current communication between on-site workers and dispatchers. This system, implemented by Metro-North after a worker was fatally struck by a train in 2013, gives a foreman at a work site a unique code. A dispatcher cannot route a train to that track without speaking to the person on site, asking for the code and entering it into a control panel.

In a response to questions sent Wednesday night, Amtrak would not say whether any of the recommended technologies were in use Sunday on that stretch of the line.

"Any technology designed to serve as a secondary warning system must be carefully evaluated," said Craig Schulz, an Amtrak spokesman. "At Amtrak, we have sought to build a culture where safety is a way for life for us and deeply ingrained in everything we do."

Rail experts, however, said they were not aware of Amtrak having any such auxiliary protections for workers.

Schulz said existing safety standards would be reviewed as part of the NTSB investigation into Sunday's deaths.

A warning system worn by workers is already being pursued by SEPTA, officials there said. These systems can be integrated into PTC so the presence of workers and vehicles on the track could automatically slow or stop a train, said Jim Resio, of Protran Technology, makers of armbands that give workers 15 to 30 seconds of warning of coming trains. SEPTA is in the process of buying the armbands for its crews.

Even a simple technology like shunting, which alters the electrical current on a rail to alert trains to an obstacle, might have made a difference Sunday, Zarembski said.

The 2014 FRA advisory was issued to all railroad agencies, but in the last 10 years the NTSB has issued two directives that also recommended the need for backup safety systems.

The NTSB directives - which were issued to transit agencies, such as SEPTA, but not to Amtrak - mirror the recommendations in the FRA advisory and were prompted by deadly accidents similar to Sunday's.

One came after a man was hit by a train while working on Washington's Metro in 2006. The other happened in 2013, when a train hit two Bay Area Rapid Transit workers in Northern California.