Stephen Gardner, speaking to Congress on Thursday, said approximately 94% of the railway’s workers were fully vaccinated, and 96% have received at least one dose, but enough remain unvaccinated that there could be notable staffing shortages next month.

“Because many engineers, conductors and onboard service employees retired or left Amtrak during the pandemic, and we temporarily halted hiring due to funding uncertainty and COVID-related distancing requirements that inhibited training, we anticipate that we will not initially have enough employees to operate all the trains we are currently operating when the federal mandate takes effect,” Gardner said.

The prepared remarks came two days after a federal judge blocked the nationwide rollout of the mandate, and it’s unclear if Gardner’s comments still reflect the railway’s situation.

The schedule changes, should they happen, would primarily impact long-distance services. Regional trains, especially along the busy Northeast corridor, aren’t expected to see any disruptions. Amtrak says it plans to restore full service and frequency by March.

“Achieving full-service levels, while complying with the vaccination requirement and continuing to prioritize the safety of our customers and employees, is our goal,” he added.

One silver lining of the pandemic, Gardner suggested, was Amtrak has become a more attractive travel option for some people, even as the total passenger count has gone down.

In recent months, 30% of the railway’s passengers made their first trip on Amtrak, he said. That’s double the pre-COVID average.