WASHINGTON — The railroad industry is playing down expectations that a safety technology that could have prevented recent deadly train crashes will be in operation across the United States by the end of the year.

Congress required in 2008 that railroads adopt the technology, known as positive train control, and gave them seven years to do the job, and then another three years. Provisions were added to the legislation to allow railroads that showed substantial progress, but couldn’t meet the new deadline, the ability to obtain extensions of up to two additional years.

Officials for the trade associations representing the seven major freight railroads in the U.S. and the nation’s commuter railroads now say they view Dec. 31 as the date by which railroads must meet several PTC milestones to qualify for an extension, rather than the ultimate deadline.

The Transportation Department has little choice but to grant the extensions as long as railroads meet the milestones, said Kathryn Kirmayer, the Association of American Railroads’ general counsel. One milestone is that freight railroads have PTC in operation on half their route miles where it’s required.

Members of Congress expressed frustration with railroads backing off this year’s deadline.

Congress never intended the extensions be used “to allow railroads that have dragged their feet to just blow off the mandate,” said Rep. Peter DeFazio of Oregon.

The National Transportation Safety Board has said the technology could have prevented a collision between an Amtrak train and an out-of-service CSX freight train on Sunday near Cayce, South Carolina, and the derailment of an Amtrak train in December near Olympia, Washington. Five people were killed and dozens more injured in the two crashes.