In this file photo, a southbound Baltimore Metro is shown from the elevated platform at the Reisterstown station. MTA announced on Sunday that the Baltimore subway would shut down for up to a month for emergency repairs. (Gene Sweeney Jr./Baltimore Sun)

Metro crews labored through an unprecedented emergency 29-hour shutdown nearly two years ago. Now track workers from the agency are lending their experience to a troubled neighbor to the north.

Metro said Monday that it sent equipment and about a dozen workers over the weekend to Baltimore to assist in the emergency shutdown of the city’s 15.5-mile MTA SubwayLink system. The agency declined to say who was paying the workers.

After a weekend shutdown for emergency repairs and inspections, the Baltimore system’s administrator abruptly announced Sunday that conditions were so dire, the entire subway would remain closed for up to four weeks, through March 11, for urgent repairs.

MTA Administrator Kevin Quinn said in a statement Sunday that the subway would need to stay closed “to ensure the safety of all customers.” The repairs are focused on deteriorated tracks, but the exact nature of the work was unknown. The MTA did not respond to multiple requests for comment, but officials said Sunday that the repairs were so critical they could not wait until summer.

The shutdown announcement elevated Baltimore’s three-decade-old subway to poster-child status in the conversation about the nation’s decaying infrastructure on the day the Trump administration announced its long-awaited infrastructure plan.

Trump’s $200 billion plan cuts a major source of federal mass transit funding. It also pushes federal officials to sell, privatize or otherwise dispose of a broad array of government assets, from Reagan National and Dulles International airports and the George Washington Memorial Parkway to properties held by federal agencies across the country.

System users will be riding shuttle buses instead. (Michael S. Williamson/The Washington Post)

Rob Puentes, president of the nonprofit organization Eno Center for Transportation, said the abrupt one-month systemwide shutdown of an urban rail system was unprecedented in the United States. Other systems, including Metro, have shut down for a day at a time. And still others have had extended line-segment closures, including a portion of Chicago’s Red Line in 2013 and Metro’s SafeTrack repairs in the Washington region.

“It’s the whole system and for a long time,” Puentes said of the Baltimore shutdown. “There’s nothing like it in the country.”

Meanwhile, Metro’s decision to send some of its workers to Baltimore to help kicked off another skirmish with its union. The two parties are in a labor dispute after contract negotiations stalled, sending them into binding arbitration last September.

Metro spokeswoman Sherri Ly said Metro offered “technical assistance” to the Baltimore system over the weekend and sent about a dozen workers to help with rail replacement efforts. Ly said the move did not affect Metro’s weekend track work or scheduled maintenance, “which continued as planned.” She said transit agencies customarily offer each other help during emergencies.

“There is a long-standing tradition of mutual aid in the transit industry, not unlike when our local electric utilities scramble resources to areas hit by hurricanes and other natural disasters,” Ly said.

But the union said the agency’s decision runs counter to its growing reliance on outside contractors to perform critical repairs for its own system. Metro estimated that about half of its SafeTrack work was performed by contractors, for example.

“Our union . . . finds it suspicious that [Metro] General Manager Paul Wiedefeld continues to bring in unqualified contractors to fix the Metro system here, while sending our qualified maintenance workers to Baltimore to fix the system he spent two years running as it fell into disrepair,” said Jackie Jeter, president of ATU Local 689, which represents most of Metro’s 12,000 employees.

“The front-line workers of Metro stand ready to assist our sister union in Baltimore whenever possible. However, at this time when Metro is in urgent need of dedicated funding to fix and maintain our system, the money allocated for Metro should be spent on Metro and not outside contractors or other rail systems,” Jeter said.

Wiedefeld served as administrator of the MTA from 2007 through 2009. Ly said Metro had no ongoing plans to send workers to Baltimore during the month-long shutdown and directed questions on the scope of the work to the MTA.

Sunday’s shutdown came as a surprise to transit observers and the Baltimore system’s 40,000 daily riders. SubwayLink, opened in 1983, consists of one line passing through the downtown core, with 14 stations linking Johns Hopkins Hospital on one end with the northwestern suburbs on the other.

A northwestern segment of the system closed in the summer of 2016 for 23 days for “critical maintenance work,” the Baltimore Sun reported. Riders, the Sun said, griped about being shuttled on yellow school buses. In response to the latest closure, Gov. Larry Hogan (R) said the state would provide $2.2 million to run shuttle buses along the subway route, which would complement existing bus lines around the stations.

Jeter said the inspections in Baltimore revealed a “desperate need” for increased funding “to be used on repair of all the transit systems in Maryland.”

Puentes, meanwhile, said Baltimore’s situation fits into a larger pattern of decaying urban rail infrastructure in the United States — between the troubled Metro and New York subway systems, extended repair work on the Chicago Transit Authority’s “L” and the Amtrak crashes that have drawn national attention to infrastructure.

“There’s clearly something alarming that needs to be dealt with,” he said. “What we’re facing is a recognition across the country of the condition [the systems are] in. The tolerance level for systems that are deemed inefficient or unsafe is very low right now, and so I think that they’ve made that calculation, ‘We’re going to go to these extreme measures and shut down the system because the alternative is unthinkable.’ ”

In addition to the subway, Baltimore has a 33-stop light-rail system serving an average of about 22,000 riders weekdays, and a bus ridership of about 250,000 weekday boardings, according to the MTA. It is the 13th-largest transit system in the country.

Though he approved the slimmed-down, 16-mile light-rail Purple Line for suburban Washington, Hogan canceled Baltimore’s planned 14-mile light-rail Red Line in 2015. The administration instead diverted those funds to road and bridge projects in other parts of the state.

Michael Laris contributed to this report.