Muni Metro riders can look forward to shiny new, and, one would hope, more reliable light-rail cars soon, perhaps by October, but first they’ll have to put up with a little pain.

Beginning Saturday, and continuing for about a month, the Metro subway will be closed between West Portal and Montgomery Street stations all day on weekends and after 9:30 p.m., instead of midnight, on weeknights. The subway will remain open Aug. 11-13 during the Outside Lands music festival. The shutdown will enable Muni to test the first five of its new light-rail cars in the subway, where they’re controlled by computers.

During the closures, Muni will use buses to run between stations on the subway portions of Metro lines, although those typically take longer and travel more circuitous routes.

Metro riders in the Powell Street station Thursday said the shutdowns would be inconvenient but worth the hassle.

“I guess I’ll find other ways of getting home if I’m staying out late,” said Matt Long, 31, a software developer who rides Metro three or four days a week. “Midnight already seems early, 9 p.m. is a lot worse. But it’s fair. I get it.”

Carl Folk, who’s 58 and self-employed, said the shutdowns and bus shuttles would make it a little tougher to get around. “But if they’re going to replace the cars, we can deal with it.”

The first of 265 new railcars arrived in January and Muni has been testing them in its yards and on the street-level sections of the Metro lines, where operators control the trains. John Haley, the Municipal Transportation Agency’s transportation director, said the tests have turned up many problems but they’ve been fixed. None is major or expected to cause delays, he said.

Workers have had to adjust the heights of some boarding platforms and move some cables to accommodate the new cars, which are being manufactured by Siemens in Sacramento, and the workers have replaced the headlights with brighter ones.

The biggest challenge has been getting the 14 different systems on the new trains — from air conditioning to Clipper readers to the public-address announcement and message displays — to work together.

Trains in the subway are computer controlled, so tests in the tunnel will focus on fine-tuning the control system to work efficiently with both the new Siemens cars and existing Breda cars operating in the subway at the same time. The control system governs the propulsion and braking systems, when the doors open and close and where and when trains stop.

Shutting the subway will allow Muni to complete the tests more quickly than if it performed them during the two hours or so available each night, Haley said.

The tests are required before the state Public Utilities Commission will certify the cars to carry passengers. Muni hopes to conclude testing by Aug. 20 and give the results to the PUC, which could take a month or more for approval. Then Muni will spend a week or so “burning in” the cars by running them for 1,000 miles, Haley said, as a final test.

Muni eventually will replace all of its existing 149 Breda cars and expand its fleet to handle expected growth in population and ridership. By the end of 2018, 24 new cars will have been delivered, with four more arriving in time for the Warriors’ fledgling arena opening scheduled for 2019.

The rest of the cars are scheduled to be delivered starting in 2021and continue to arrive gradually through 2028. MTA officials are talking with the Federal Transit Administration about possibly speeding up that process.

The first three cars pressed into service, said Haley, will be placed on the crowded and delay-ridden N-Judah line, where he expects them to have a quick impact.

“We expect leaps and bounds improvements with reliability,” he said. “Service will improve immediately.”

The cars, which are gray and white and wrapped with a big red stripe, will feature more dependable doors, more efficient braking, a more spacious interior and more stanchions and handholds to grab, he said. People who’ve seen them in testing on the streets, Haley added, are excited about the new cars and often have to be prevented from boarding.

“If that’s any indication,” he said, “people will be thrilled with the new trains.”

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanataun@sfchronicle.com Twitter: @ctuan