The U.S. Federal Railroad Administration on Thursday ordered Amtrak to conduct a safety review after Sunday's passenger train accident near Philadelphia that killed two people.

A Georgia-bound train struck a backhoe on the tracks, killing the backhoe operator and another Amtrak construction worker on foot and sending 35 people to hospital.

The accident took place about 20 miles south of where another derailment last May killed eight and injured 43.

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The railroad administration and the National Transportation Safety Board are investigating.

In the meantime, the administration directed Amtrak to conduct a "safety stand down" for track maintenance workers, which it described as a comprehensive review of all safety rules with employees.

Amtrak said it was working with the safety board to identify the cause of the accident and vowed to make changes if needed.

"Amtrak has strict protocols that crews must follow when tracks are taken out of service for maintenance or when work is being performed along the right-of-way," spokeswoman Kimberly Woods said in a statement.

"Amtrak has a strong safety record. ... However, we need to assess how we can get better," Woods said.