Amtrak has temporarily dropped its mandate for all employees to be vaccinated against Covid-19, a requirement that the rail service had said would likely result in cuts to service.

As a result, Amtrak no longer plans to cut service next month, officials said on Tuesday.

William J. Flynn, the chief executive officer of Amtrak, said in a memo that the agency would allow employees to opt into weekly testing instead of receiving a vaccine, reverting back to a previous policy. About 97 percent of Amtrak’s employees have either received a shot or an accommodation providing an exemption.

“Recently a federal district court decision halted the enforcement of the executive order for federal contractors,” Mr. Flynn said. “This caused the company to reevaluate our policy and to address the uncertainty about the federal requirements that apply to Amtrak.”

The shift in policy means that Amtrak officials no longer anticipate cuts to service next month, Mr. Flynn said. Last week, Stephen J. Gardner, the president of Amtrak, said that the agency expected to scale back service because 5 percent of its work force had yet to receive a vaccine ahead of a Jan. 4 deadline.

Less than 500 employees have not been vaccinated, Mr. Flynn said. Employees who fail to comply with the new policy will be placed on an unpaid leave of absence.

Amtrak officials said they will continue to update the agency’s vaccine policies “as needed,” according to the memo.

Still, Mr. Flynn encouraged all employees to get inoculated, especially as the nation braces for a potential wave of cases from the Omicron variant.

Jim Mathews, the president and chief executive officer of the Rail Passengers Association, said he would prefer that all of the agency’s employees to be vaccinated, but that Amtrak had made substantial progress so far.

“This addresses the issue without compromising safety,” Mr. Mathews said in a statement. “Everyone, crews and passengers, is safest if everyone is vaccinated. But Amtrak reached huge numbers of vaccinations within their staffs, and that’s to their credit.”