WASHINGTON - U.S. lawmakers on Thursday warned that Congress must act well before year-end to extend a Dec. 31 deadline for railroads to adopt new safety measures, to avoid major disruptions to freight and passenger service, but some said they are uncertain about how to act. “We cannot wait until the last minute to act,” Senate Commerce Committee Chairman John Thune said at a hearing. Without congressional action, Thune said, railroads will probably “begin to cycle traffic off their lines” four to six weeks prior to Dec. 31, the congressionally mandated deadline for implementing safety technology known as positive train control, or PTC. Most railroads are not expected to meet the deadline, and major freight handlers have threatened to suspend service rather than operate outside the law if no extension is granted. Last month, the Senate approved a bipartisan measure that would allow the Obama administration to extend the deadline for up to three years. But it is not clear whether the House of Representatives will accept that approach. PTC is a complex communications system that can automatically slow or stop a train to avoid derailments and major crashes. Federal safety officials say it would have prevented the May 12 Amtrak derailment that killed eight people and injured more than 200 others. But railroads say implementation efforts have been snarled by high costs, bureaucratic delays and technical hurdles. Some lawmakers called on the Federal Railroad Administration (FRA), the government’s chief rail regulator, to provide guidance on what kind of extension to grant and for how long. The hearing was held to consider acting FRA Administrator Sarah Feinberg’s nomination to become full-time agency chief. “I would appreciate a specific recommendation as to the length of time that might be appropriate. Is it six months? Is it a year? Is it two years?” Senator Roger Wicker, a Mississippi Republican, told Feinberg. Feinberg said FRA hoped to work with individual railroads to phase in PTC operations rather than face a new blanket deadline. But she rejected a recommendation from the Government Accountability Office urging lawmakers to allow regulators to change deadlines on a case-by-case basis. “I am anxious about the prospect of entering into negotiations with 40 different railroads on a case-by-case basis, which would result in a choose-your-own deadline back-and- forth,” Feinberg said.