Protesters rally against the government shutdown at the U.S. Capitol building on Jan. 10. (Alex Wroblewski/Bloomberg)

Metro officials are concerned the government shutdown is cutting into the agency’s revenue as the system that carries a sizable share of the federal workforce runs full service despite thousands of employees staying home on furlough.

If the shutdown persists, Metro board members said, the agency may be forced to ask local jurisdictions for more money to support its operating budget, scale back service to recoup some of its costs or find savings elsewhere.

Metro has declined to release data on the impact of the shutdown; officials say they want to gather a fuller picture before issuing the information. But internal data available to agency staff and obtained by The Washington Post show ridership this week was down about 50,000 trips per day on average through Wednesday, with decreases ranging from 6 percent to 11 percent compared to the same week last year. It was unclear how much of the losses could be attributed directly to the shutdown because year-over-year ridership is not an exact comparison.

“We already know anecdotally it’s been significant,” said Metro board member Christian Dorsey, chairman of the panel’s finance committee. “I’m just hoping that once we total it up we don’t have our hair on fire because it’s so huge — and then the longer it goes the more it’s magnified.”

For Metro, which services numerous federal hubs, including L’Enfant Plaza, Medical Center and Pentagon, the impact of an extended shutdown could be acute. During the 2013 government shutdown, Metro ridership dropped more than 20 percent, and the agency was forced to run shorter trains to meet the reduced demand. The two-week shutdown cost the agency more than $5 million, according to media reports.

The agency has estimated federal workers make up 40 percent of its rush-hour ridership.

Meanwhile, this shutdown was slated to become the longest in history over the weekend, surpassing the 21-day shutdown between 1995 and 1996. However, because the beginning of the shutdown fell during what is typically a lighter ridership period — during the Christmas holiday — the immediate impact was not expected to break Metro’s $1.8 billion operating budget.

But there were hints this week that the shutdown was beginning to make a dent.

“It’s how long does this go and what does that mean?” Metro General Manager Paul J. Wiedefeld said as the shutdown stretched into its third week. The effects go beyond ridership, he said.

Wiedefeld said the shutdown raises numerous concerns for the transit agency and the region: the revenue challenges, the disbursement of federal grants to pay for capital projects, and a looming April deadline for federal certification of the Metrorail Safety Commission, the body set to assume safety oversight from the Federal Transit Administration.

While the FTA’s Metro safety inspectors remain on the job — they are deemed “essential” — the processing of grant funding the agency is owed has stopped.

Only 11 percent of FTA employees are working during the shutdown, according to the U.S. Transportation Department’s shutdown plan. There are seven employees responsible for Metro oversight who will continue to respond as needed. Several contractors assigned to Metro oversight also remain on the job and are being compensated from unexpired appropriations from previous years.

“We won’t have a cash flow issue but it does eventually — potentially — have an impact,” Wiedefeld said of the federal grant money. “None of us know how long this goes. Who knows?”

Meanwhile, if the safety commission is not in place by the April deadline, according to the Metropolitan Washington Council of Governments, “FTA will be prohibited by law from obligating any federal transit program funds to any public transportation agencies in the District of Columbia, Maryland and Virginia” until its certification.

The FTA conducts the certification process.

The FTA says it would be “prohibited by law” from issuing a total of $638 million in federal transportation funding to the region until the commission is certified.

“That one is just in the back of my mind,” Wiedefeld said. “As time goes by that could potentially be a bigger issue. Right now it’s not, but again no one knows how long this goes.”

And if the ridership loss results in an operating shortfall?

“We’d have a discussion with the jurisdictions basically to make up those dollars,” Wiedefeld said.

Dorsey, who represents Northern Virginia, said regional anxiety over the issue mounted as the likelihood of a closure increased. A shutdown-related funding shortfall would complicate things for the agency because it is pushing for additional money to pay for service increases.

“And the longer [the shutdown] goes on the worse that it gets,” Dorsey said. “We’re about to head into uncharted territory. I guess in a few days it’ll be the longest shutdown in history. This will all be uncharted territory.”

Other board members were skeptical that the effects would be so dramatic.

“In my estimation I think there could be some modest revenue impacts from lost fares, but probably not anything that is beyond the ability of management to compensate for in the overall operating budget,” said board member Steve McMillin, a Trump administration appointee who represents the federal government.

McMillin was encouraged that, unlike past shutdowns, about half of federal employees are working — though some without pay. Additionally, more employees recently returned to work when the administration ordered the Internal Revenue Service to process tax returns and issue refunds.

Board member Michael Goldman said Metro was just beginning to feel the effects of the shutdown, though he did not notice it on his packed Red Line train to Thursday’s board meeting. It was “chock-a-block full,” he said, adding it might have been a “fluke.”

“If there’s less ridership there could be a shortfall,” said Goldman, who represents Maryland. “It’s the long-term impact if this continues for 30 or 45 days,” he said noting his biggest concern.

Metro is set to issue a detailed analysis on the shutdown’s impact next week.

“If this goes very, very long that’s a totally difficult equation, but we’re just not there yet,” Wiedefeld said.

Lori Aratani contributed to this report.