BART riders aren’t the only transit passengers who will soon get to enjoy that new railcar smell for a while. By the end of the year, Muni expects the first of its $1.2 billion fleet of new Metro cars to start rolling into the city.

After testing, the light-rail cars, being assembled at a plant in Sacramento, will probably be pressed into service by summer 2017. The new cars are part of a plan to grow and modernize the Muni Metro fleet, which is overwhelmed by high ridership and the unreliability of the existing Breda railcars, purchased in 1996.

With a larger fleet, Muni will be able to run longer trains — three or four cars long — and introduce express trains that would skip certain stations. To make ridership more efficient, Muni also plans to turn some trains around before the end of its lines to increase the frequency of service to congested areas.

“We will be entering into a new era in 2017 as far as providing service,” said John Haley, the Municipal Transportation Agency’s transit director. “With more cars, we can start having some fun, trying some new things.”

The MTA board has ordered 260 new railcars to replace the 151 cars it now runs on the six Metro lines that travel through the Market Street subway. The first pair of new cars is expected to arrive this fall. They will undergo about 600 tests, and drivers will be trained, before the vehicles are pressed into service.

In preparation, Muni is completing some infrastructure improvements that will allow the cars to run more smoothly. The agency is building a training simulator, creating training procedures and stocking up on parts to maintain the new cars.

Meanwhile, in Sacramento, workers at the Siemens plant are putting together the frame and exterior of the first new Metro car. Siemens is a German corporation. There are no U.S.-owned rail manufacturers, but Buy America rules require that the railcars be assembled in the United States.

By 2018, Muni will have 24 new cars in time for the scheduled opening in 2019 of the Central Subway. Another 40 will be ready to roll by 2020. Muni plans to hold onto its existing Metro cars until 64 of the new vehicles are on hand, then will begin retiring them. The new cars are intended to handle ridership growth through 2040.

“We took a long view,” Haley said. “We’re not just replacing the fleet we have; we’re looking toward the future.”

The interior, including the seating configuration, is still being designed, but Muni riders can expect better air conditioning and ventilation systems, wider aisles, brighter lighting and doors that don’t break down nearly as often.

“It will be a quantum leap forward in terms of reliability,” Haley said.

Malcolm Heinicke, an MTA director, said the changes, especially longer trains, will make a big difference for commuters stuck on overcrowded trains that are sometimes so packed they bypass stations because there’s no room for more passengers.

“This is exciting stuff, and stuff that can change how the system interacts with passengers on a daily basis,” he said.

Muni Metro riders, waiting for a one-car train at Civic Center Station Tuesday evening, welcomed the thought of longer trains.

“That sounds good. It should help,” said Natalie Herry, 26, a student who lives in the Tenderloin.

Another change that will help Muni Metro is coming sooner.

On Monday, Muni started testing double stopping, a process that allows trains serving Civic Center, Powell and Montgomery stations to stop and let passengers get off the train before moving forward to pick up passengers. So far, the tests are working, said Ed Reiskin, MTA’s transportation director, adding that Muni Metro is likely to continue double stops — at least for inbound trains in the morning commute.

Yevgeni Kedrun, a salesman who immigrated to San Francisco from Belarus eight months ago, likes Muni but said he’s glad to see the agency experimenting with ways to improve service.

“You always have to change,” he said. “Without trying something new, you never improve.”