Investigators said Amtrak had a "weak safety culture" where employees frequently took short cuts and put on-time performance over safety.
While drug use did not have a "direct causal link to this accident," according to investigators, it is a reflection of a lax safety culture at Amtrak, they said.
The Federal Railroad Administration revised its federal drug testing rules to include Maintenance of Way workers, like those who were operating the backhoe, effective on June 12, 2017.
Previously DOT regulations only required drug testing for locomotive engineers, trainmen, conductors, switchmen, locomotive helpers, utility employees, signalmen, operators and train dispatchers.
These rules establish minimum requirements for drug testing, but rail companies are free to expand them.
On Monday the FRA said it was adding certain semi-synthetic opioids such as hydrocodone, hydromorphone, oxymorphone and oxycodone to its testing.
Railroad repairs were ongoing in the days leading up to the fatal accident. A night foreman was found to have lifted a track safety closure while a backhoe remained on the track. The day foreman did not restore the closure, according to investigators, leading to a train striking the backhoe at nearly 100 mph.
NTSB chairman Robert Sumwalt described the mistake by the foremen as "the fundamental error of the night."
Among the contributing factors to the crash were lack of communication between employees, improper establishment of work zones and pressure from managers to keep trains on time.
Positive Train Control, a federally-mandated automatic train-braking system, had been installed in the Northeast Corridor where the crash occurred, but investigators said a series of human errors, such as not properly establishing the work zone, circumvented the technology.
Despite Amtrak requiring the use of shunts, a device that could have signaled the track was occupied at the time, investigators said the maintenance workers did not have them at the accident site.
Amtrak has since purchased thousands of the devices.
Amtrak spokeswoman Kimberly Woods told ABC News that Amtrak has "taken a series of actions to improve workplace safety at Amtrak," but did not respond when asked what exactly those actions are.
The engineer of the involved train was fired following the accident, according to the NTSB, due to his toxicology report.
Woods would not say if the foremen who mistakenly left the occupied track open were still working for Amtrak. She says the company does not comment on personnel matters. Amtrak is a government-funded company that operates as a for-profit corporation.