The San Francisco Municipal Transportation Agency [SFMTA] warns that for approximately two weeks starting August 12, train service through the subway will shut down early each night, leaving late-night commuters to rely on replacement buses to make trips between Embarcadero and West Portal Stations.

SFMTA dropped the announcement last week, noting that from 9:30 p.m. to 1 a.m. bus substitution service will replace light-rail runs.

After 1 a.m., regular Owl-line service will kick in like usual, and “trains will start regular morning service” through the underground every morning on the usual schedule.

While the bus substitutions mean no rider will be left stranded at night without service, SFMTA notes that “Muni customers are encouraged to leave extra travel time when riding Muni during subway closure.” Longtime riders know that adding shuttles and trying to synch up schedules between multiple vehicles almost always leads to extra delays and uncertainty.

The nightly shutdown is meant to give Muni extra time to fix nagging problems in the tunnels.

“The extended window will give us an opportunity to do complex work […that] will cover overhead lines, tracks, signals and the Automated Train Control System,” the transit agency announced earlier this year, adding that a similar shutdown will kick in come December.

Muni’s light-rail service thus far in 2019 has been mediocre at best. In June, the overall on-time rating for the six trains that utilize the subway (counting the T-Third Street) was just 46 percent.

That’s up significantly from 39 percent January and 41 percent in June of 2018 but still pretty disappointing in any other context.

Trains were early (arriving more than one minute before scheduled) 15 percent of the time, late 22 percent of the time, and “very late” (arriving ten minutes or more after schedule) 18 percent of the time.

Here’s how each line performed for the month:

• K/T: On-time 38 percent
• J-Church: On-time 39 percent
• M-Ocean View: On-time 45 percent
• L-Tarval: On-time 51 percent
• N-Judah: On-time 57 percent