Muni Car No. 2068 trundled down Third Street in San Francisco’s Bayview district, with the slow-motion elegance of a parade float.

It’s the last car to roll out in Muni’s first batch of new light-rail vehicles, the red-and-gray Siemens trains that will gradually replace the old Breda fleet. Since debuting them last year, the transit agency had to grind through a series of problems, including doors that closed on people’s hands, broken couplers, and brakes that caused the wheels to flatten.

Engineers at Muni and Siemens addressed each of these flaws, said Emmanuel Enriquez, test supervisor for the new vehicles. The doors now have additional sensors that bleep and blink when a person places a hand or pinkie finger into the doorjamb, preventing the door from closing. During the testing process, fleet inspectors filled small gloves with sand to replicate the tiny hands of children.

To fix the wheel problem, Muni placed track brakes between each set of wheels to create a “Fred Flintstone” effect, Enriquez said — similar to the cartoon character dragging his feet to stop a car. Train mechanics also redesigned the couplers so the agency can link cars and manage crowds in its subway system.

In the coming months, Muni will also replace the slippery bench seats in its new cars with a “scalloped” version that includes bucket-shaped indentations for each rider’s rear end. Cranky passengers scorned the bench seats as soon as they appeared, saying they awkwardly pack people together and cause injuries for riders with back problems.

Muni’s new seat configuration will place a bench at one side of each car and rows of forward-facing seats at the other.

“We never stop engineering,” Enriquez said.

Car 2068 is still undergoing testing, an arduous process in which operators drive the train 1,000 miles, logging every problem that comes up before it’s allowed to hit streets. Trains and buses in San Francisco have to navigate traffic-snarled streets and make hairpin turns on steep hills. Such movements could “rip a train apart,” if it’s not designed correctly, Enriquez said.

On Tuesday, train operator Lisa Telsee drove 2068 for half the length of Muni’s T line, from the Muni Metro East yard at 25th and Illinois streets, to Sunnydale, a section of Visitacion Valley just south of John McLaren Park. Along the way, the train passed rows of small grocery stores, churches and scruffy palm trees. It faced one tough incline on the way back, near the Bayside Cafe, Enriquez said.

Currently, Muni’s fleet is a multigenerational hodgepodge, with 68 glossy Siemens trains, 148 Bredas and 60 historic streetcars. When the San Francisco Transportation Municipal Transportation Agency starts the next phase of its $1.2 billion procurement in 2021, officials will retire one beaten-up Breda for every new Siemens.

The old cars, tinny and gray, with analog destination signs, could meet the same fate as BART’s legacy trains. Most of them will be gutted and scrapped, though some may enjoy second lives as Burning Man art cars or rail museum show pieces. They might even be repurposed for transit systems in other countries.

“They could become barbecue pits, for all we know,” Enriquez said.

He anticipates a full swap in six or seven years. At that point, 219 deftly engineered Siemens cars will carry passengers to all pockets of the city.

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com