A Metro Silver Line train derailed just outside the East Falls Church station on July 29, 2016. Metro has revised its SafeTrack maintenance plan to address problems uncovered since then. (Nikki Kahn/The Washington Post)

Metro General Manager Paul J. Wiedefeld on Tuesday announced a revised SafeTrack plan that adds several weekend shutdowns and increases the likelihood that the 10-month maintenance program will last longer than expected.

Officials also said that the “safety surge” now underway and slated to end Thursday will be extended through Sunday.

Metro said the revised plan is necessary for aggressive work to be performed on interlockings — the points where trains change tracks — to address new federal safety recommendations, and to account for the impact of excessively hot weather and high humidity, which have slowed the work.

The changes will have the greatest impact on Virginia riders, who will experience the added weekend shutdowns for the interlocking work made necessary after a derailment last month raised questions about the safety of Metro’s track infrastructure.

The schedule shift had been expected since last week, when Wiedefeld said at a news conference that the July 29 derailment at East Falls Church had led Metro officials to consider changes to the SafeTrack plan, particularly to tackle six interlockings — the same track infrastructure that caused the derailment of a Silver Line train.

Metro Board Chairman Jack Evans said Wiedefeld briefed him on the schedule adjustments early Tuesday, saying the excessive heat and newly uncovered problems both factored into the decision.

“It’s hot, and so, given the weather problem, he decided, rather than burn everybody out, to give everybody another week or so,” Evans said, adding the changes would extend the length of SafeTrack by up to two weeks and possibly longer.

He said Metro officials knew from the outset that SafeTrack would have to be adjusted as the plan proceeded.

“It was anticipated by us that we were going to find things that we didn’t know were out there,” Evans said. “And I would anticipate we find more things as we go along.”

The service disruptions would be unwelcome news for riders. But, he said, “we’ve gotta get this thing fixed.”

Even so, the new schedule caught Virginia commuters and public officials unawares.

Fairfax County officials did not get much in the way of advance warning about the last-minute changes to the schedule — changes that will require some quick reshuffling of personnel who had been set to staff additional shuttle buses starting this Saturday for the next surge.

“We’re just trying to keep up,” said Beth Francis, spokeswoman for the Fairfax County Department of Transportation. But, she added, “we’ll work it out.”

The seventh SafeTrack surge, which is underway on the Red Line between Shady Grove and Twinbrook, will be extended through Sunday to accommodate a new weekend shutdown. The work was originally scheduled to conclude Thursday.

Beginning Saturday, free shuttle buses will replace Red Line trains between Grosvenor and Shady Grove as Metro closes four stations: White Flint, Twinbrook, Rockville and Shady Grove.

Red Line trains will continue single-tracking between Shady Grove and Twinbrook through Friday.

Surge No. 8, originally scheduled to begin Saturday, will be pushed back to Aug. 27 and will continue through Sept. 11. In addition to weekday single-tracking from Franconia-Springfield to Van Dorn Street, the surge now encompasses three consecutive weekend shutdowns as crews work to rehabilitate a switch in the area.

On Aug. 27 and 28, Franconia-Springfield station will close and shuttle buses will provide service to Van Dorn Street. Then, from Sept. 3 through 5, including Labor Day, shuttle buses will replace trains from Franconia-Springfield to King Street. The Franconia-Springfield and Van Dorn Street stations will be closed.

Franconia-Springfield will again close Sept. 10 and 11, with shuttle buses to Van Dorn street, as the work concludes.

The ninth SafeTrack surge, which includes daily single-tracking between Vienna and West Falls Church, will be pushed back to Sept. 15 and continue through Oct. 26. It will also include four new weekend shutdowns: Sept. 24 and 25, Oct. 1 and 2, Oct. 8 and 9 and Oct. 15 and 16. Vienna and Dunn Loring stations will be closed for all four shutdowns. West Falls Church will be closed for the first two.

Delays attributed to hot weather aren’t new to SafeTrack: Heat was an issue during Surge No. 3, in early July, when officials said that workers were unable to complete all of their planned work on drain maintenance, grout pad renewal, sign repairs and traction power systems because of the need for workers to take frequent water breaks to ward off heat exhaustion.

Metro Board member Christian Dorsey said he is frustrated at that reason for the delays. He said he wants answers on why the effects of heat and humidity — inherent to any construction project happening in the middle of summer in the Washington region — were not built into the initial schedule.

“It’s not inconceivable that this was going to happen,” Dorsey said. Still, he said, “you would think that when planning out a project of this scope, weather would be an issue that you’d expect.”

Even so, Dorsey said he is glad that Metro officials are focusing on using each surge to maximum effect.

“It’s about really making sure that once you’re going in and doing extensive track work, you’re doing everything that needs to be done,” said Dorsey, who represents Virginia on the board. “That is acceptable to me.”

The silver lining? Surge No. 13, a complete shutdown on the segment of the Blue Line between the Rosslyn and Pentagon stations, had previously been scheduled during the 18 days leading up to Christmas.

Now, Dorsey mused, maybe the surge will get pushed back until after the prime holiday shopping season.