WASHINGTON - U.S. railroads that were granted a three-year delay to install a lifesaving safety mechanism have been told by federal regulators that they will be held to a strict schedule to ensure they get the job done.

The device - positive train control - has been called "arguably the single-most-important rail safety development in more than a century" by federal regulators.

Safety experts say PTC could have prevented the deaths of eight people in a Philadelphia Amtrak derailment this year, and those of 288 other passengers and train crew members since 1969.

They consider implementation of PTC more important than ever since trains filled with volatile natural gas or oil have derailed nine times so far this year, and they fear one could cause catastrophic explosions as it passes through a city.

With the boom in domestic oil production, the number of rail tank cars carrying flammable material in the United States has grown from 9,500 seven years ago to 493,126 last year.

The railroads, most of which had fallen far behind schedule in meeting a Dec. 31 deadline for installation of PTC, petitioned Congress for relief and won a three-year extension. Congress, whose members have received more than $24 million in campaign contributions from the railroad industry since 2008, also said railroads could ask for up to two more years after the 2018 deadline to complete the job.

But the legislation that granted the extension came with teeth, and Federal Railroad Administrator Sarah Feinberg has served notice that she intends to use them.

"The focus has to remain on getting PTC up and running and implemented as soon as possible," she said in a speech to a railroad conference last week. "I recognize that the legislation allows 2018 to be the goal and there to be a potential extension beyond 2018 to 2020. But the deadline is 2018. If you need to get to 2020, there are certain boxes that have to be checked in order to get there."

Warning against expecting another deadline extension, Feinberg sent a letter to the railroads outlining the "boxes" that must be checked in the coming months and years.

She said each railroad must submit a detailed plan before the end of January for PTC implementation, with justifications for the pace at which the railroad intends to proceed, and annual reports thereafter.

Feinberg wrote that extensions beyond the 2018 deadline would be considered only if a railroad had demonstrated a good-faith effort to comply with the mandate.

The railroads long had argued that PTC technology was too complex to allow them to meet the December deadline and that the $14.7 billion cost to equip freight and commuter lines was prohibitive. They say they have devoted time, energy, and billions of dollars to installing the systems and are firmly committed to proceeding as swiftly as possible.