An earlier version of this story incorrectly said that Transportation Secretary Elaine Chao could not immediately be reached for comment. The story was published before The Post tried to reach Chao. This version has been corrected.

Amtrak investigators inspect the scene of the deadly train crash in Chester, Pa., on April 3, 2016. The Amtrak train struck a piece of construction equipment just south of Philadelphia, causing a derailment. (Michael Bryant/Philadelphia Inquirer/AP)

Two Amtrak maintenance workers had opioids or cocaine in their systems when they were struck and killed south of Philadelphia last year by a passenger train whose engineer had marijuana in his system, according to federal officials.

The revelation and the nationwide opioid epidemic led a group of House Democrats on Tuesday to ask Transportation Secretary Elaine Chao to require transportation industry workers — truck drivers, bus drivers and railroad operators — to be tested for four prescription opioids.

Transportation workers are governed by testing protocols established in 1989 to check for substances such as marijuana, cocaine and phencyclidine, commonly known as PCP.

“Yet we are in the midst of a prescription opioid crisis in America,” said a letter signed by seven Democratic members of the House Transportation Committee: Reps. Peter A. DeFazio (Ore.), Rick Larsen (Wash.), Michael E. Capuano (Mass.), Grace F. Napolitano (Calif.), John Garamendi (Calif.), Hank Johnson (Ga.) and D.C. Del. Eleanor Holmes Norton.

On April 3 last year, a southbound Amtrak train carrying 330 passengers and seven crew members, under the control of Alexander Hunter, crashed into a backhoe in Chester, Pa.

The train, traveling at 106 mph, killed two track workers, Joseph Carter Jr., 61, and Peter John Adamovich, 59.

The engine derailed but remained upright.

Hunter and 40 people on board were taken to hospitals with minor injuries.

The preliminary report by the National Transportation Safety Board said that Hunter, who was 47 at the time and had been an engineer for New Jersey Transit and Amtrak for 17 years, had marijuana in his system. It said Carter had cocaine in his system, and Adamovich had taken unprescribed oxycodone.

In January, as the Obama administration prepared to leave office, it proposed a rule that would allow the Department of Transportation to test for four prescription opioids: hydrocodone, hydromorphone, oxymorphone and oxycodone.

“Comments were due more than six months ago, and the [proposed rule] has since languished in this administration,” the House Democrats said in the letter to Chao. “We strongly urge you to take action now to finalize this rulemaking as a first step toward addressing the opioid crisis.”

The positive drug test of all three involved in the Chester crash reflected an alarming increase in drug use by railroad workers that was documented last year by the Federal Railroad Administration. Nearly 5 percent of workers involved in crashes in 2016 were found to have used illegal drugs.

After several years in which heroin and illegal opioid use had increased in the general population, it was evident that use of those and other drugs was on the rise in the railroad industry.

In 2014, no one tested positive for drug use after a rail accident. In 2015, there were two post-accident drug positives.

Railroad workers are among the most heavily drug-tested employees in the country, faced with drug screening before they are hired, random on-the-job testing and an additional round of testing every time they make a significant mistake.

In about 50,000 random tests each year, there had been no appreciable increase since 2009. But that changed abruptly in 2015: Random tests of railway workers — including engineers, train crew and dispatchers — found drug use had soared by 43 percent.

The popularity of illegal prescription drugs and heroin has increased dramatically in recent years, with some analysts suggesting that efforts to crack down on illegal prescriptions have encouraged addicts to use heroin instead.

More than 52,000 people in the United States died of drug overdoses last year, according to the Centers for Disease Control and Prevention, which predicted that 71,000 will die this year. The CDC said that 11.8 million people in the country abused opioids last year.

Despite drug-testing protocols, transportation workers appear to be as susceptible to drug-use trends as the rest of society.

The DOT drug-tests about 7 million people who hold commercial driver’s licenses, as well as railroad and transit workers, and the U.S. Coast Guard.

In the past five years, DOT tests have shown sharp increases in the use of amphetamines and natural opiates. The rule proposal advocated by the House Democrats would require testing for synthetic opioids.