The year-long Metrorail rehabilitation plan includes 15 projects that will require the longest stretches of single-tracking and station shutdowns.

Metro investigators believe that Friday’s derailment at the East Falls Church station was caused by a defect known as “wide gauge,” in which the two sides of the tracks are too far apart and cause the wheels to lose contact with the rails.

The suspected defect caused a flurry of unplanned inspections over the weekend and brought added scrutiny as the latest SafeTrack maintenance surge began Monday on the Red Line between the Takoma and Silver Spring stations.

Last week’s derailment occurred while a train was passing through an interlocking, the mechanism that allows trains to shift between the tracks. Because this round of the SafeTrack surge involves continuous single-tracking between the two eastern end stations, switches are working overtime to shuttle trains in either directions.

That led General Manager Paul J. Wiedefeld to direct staff to inspect the interlockings at Silver Spring, Takoma and NoMa-Gallaudet stations before starting service Monday morning.

The sixth surge in Metro’s long-term maintenance overhaul known as SafeTrack runs August 1 through 7 on the Red Line. Take a look at how it will affect your commute. (Claritza Jimenez/The Washington Post)

Despite the initial worries, the sixth SafeTrack surge went off relatively smoothly: Metro reported that Red Line ridership was about 30 to 40 percent down, and passengers reported encountering the same amount of crowding on trains that they would experience on an average Monday morning — even though the wait was a little longer, with a reduced number of trains running.

When Carmen McDonald arrived at the station, she had just missed a train by a few moments and realized it would be a 12-minute wait for the next arrival.

“That’s good enough for me,” she said.

LaShawn Frederick, 34, was less upbeat when she saw a 14-minute wait for an inbound train.

“That’s a long time,” she said, to no one in particular, noticing the wait times on the information display. “I’m going to have to start showing up earlier if I don’t want to be late for work.”

Preparing for the worst, Montgomery County had provided extra Ride On buses Monday. An official at Silver Spring station said there were few lines for the shuttle service through the morning commute, but at least a couple of buses left with all seats occupied, from Silver Spring to Fort Totten.

“Things seem to be going very smoothly,” Esther Bowring, a Montgomery County transportation spokeswoman said in an email. “The traffic seems lighter than normal.”

It’s early, but the District did not see “acute traffic congestion” as the latest maintenance plan got underway, according to District Department of Transportation ­Director Leif Dormsjo, suggesting that extended parking restrictions freed up some additional space on roads and helped keep cars moving.

Maryland’s commuter train system saw a slight jump in ridership Monday morning. About 220 passengers above normal commuted on MARC’s Brunswick Line, Maryland Transit Administration spokeswoman Sandy Arnette said. MARC added four rail cars to various Brunswick Line trains to handle additional riders Monday, but officials said the extra cars weren’t needed.

“We’ll keep the cars on through tomorrow in case we get more passengers on Day 2,” Arnette said.

Metro officials were cautiously optimistic that their warnings about SafeTrack maintenance had been heeded. They were even able to do additional work because of the weekend service disruption brought on by last week’s derailment.

According to Metro, workers were able to use the weekend closure of East Falls Church station to replace 450 wooden rail ties that had not been scheduled to be tackled under the SafeTrack project repairs that were already underway in that area.

Accident investigators — who included Metro staff, representatives from the Federal Transit Administration and the National Transportation Safety Board, as well as outside consultants — don’t believe that operator error was a factor in the derailment. The mishap resulted in one minor injury and caused the 60 passengers to be evacuated onto the tracks.

Metro said car equipment, weather conditions, heat and other factors may have played into the derailment just outside the station. A preliminary assessment showed that the tracks were too wide to support the train because of deteriorated rail ties, the wooden slabs that secure the running rail.

Wiedefeld ordered special inspections of all the system’s tracks “to look for any other similar conditions that must be immediately addressed,” he said in a statement.

It’s not the first time that “wide gauge” has caused problems on Metro: It’s one of the most common causes of derailments, and it’s the same problem that allowed a non-passenger train to come off the tracks last August as an operator was preparing to start service at the Smithsonian station.

Luz Lazo, Lori Aratani, Mike Laris, Dana Hedgpeth, and Robert Thomson contributed to this report.