Emergency crews respond after an Amtrak train struck a backhoe April 3 in Chester, Pa., killing two Amtrak maintenance workers. (Dominick Reuter/Reuters)

Four days after a pair of Amtrak maintenance workers were killed when a train slammed into their backhoe, federal regulators have ordered the passenger rail line to conduct a thorough safety review of work-crew rules and regulations.

The mandate issued by the Federal Railroad Administration on Thursday is the strongest indication to date that investigators think that the two men would not have died in the wreck just south of Philadelphia if federal regulations had been observed.

In addition to reviewing safety rules with workers and their supervisors, the FRA also ordered Amtrak to focus on the effectiveness of communication between work crews and the dispatchers who control train movement.

Amtrak also is required to review “cellphone use near and on tracks.”

The two men struck by the southbound train were identified by the Delaware County medical examiner as Joseph Carter Jr., 61, and Peter John Adamovich, 59. The medical examiner said Carter was operating the backhoe.

The lead investigator for the National Transportation Safety Board said that a camera on the train captured images of the work site as it approached.

“The video showed there was construction equipment on the track and work train equipment on the track immediately adjacent to the Amtrak train’s track,” NTSB investigator Ryan Frigo said. “The backhoe was on the track where the Amtrak train was traveling.”

He said that the train was going 106 miles per hour in a 110-mph zone and that the train’s operator slammed on the emergency brake five seconds before it hit the backhoe.

Amtrak President Joe Boardman, a former FRA administrator, said he welcomed the stand-down orders.

“We agree with the FRA directive and are moving to immediately take action,” he said. “A safety stand down is a reinforcement tool we have used at Amtrak in the past to draw immediate attention to and reinforce understanding of an issue that we believe has the potential to affect the safety of the railroad or our employees. The FRA directive helps us impart the seriousness of following our rules to prevent accidents, injuries and deaths.”

Although the investigation continues, federal regulations show that the two workers probably ignored one or more safety regulations that could have saved their lives.

Among them:

●Work crews can trip a red signal — halting an approaching train — by cutting off a low electrical current in the rail. Known as shunting, the crews clamp a device on the track that halts the flow of power, indicating that the track is occupied.

●Workers are required to perform as teams, with one member keeping watch for approaching trains. That member, the regulations stipulate, “shall not be assigned any other duties while functioning as watchmen/lookouts.”

●Work crews are to notify a dispatcher when they prepare to undertake a task on the rail bed. “The train dispatcher or control operator shall not permit the movement of trains or other on-track equipment onto the working limits . . . until the roadway worker . . . has reported clear of the track,” the regulation states.

The train departed Philadelphia’s 30th Street Station about 18 minutes earlier and had a straight run of more than two miles before it reached the site where it collided with the backhoe.