As San Francisco’s transit chief prepares to step down, Mayor London Breed and other City Hall politicians are taking ownership of the city’s stumbling light-rail and bus system.

To that end, the mayor has formed a transit performance working group in collaboration with two supervisors and the city controller. The idea: review Muni’s service and figure out how to improve it, while also taking a hard look at the sprawling, sometimes unwieldy agency that governs all aspects of transportation in San Francisco.

Breed has begun a national search to replace Ed Reiskin, who will leave his post at the helm of the San Francisco Municipal Transportation Agency in August. Yet the mayor and other policymakers are also under pressure to reshape San Francisco’s intricate transportation system themselves.

“Over the last year, the Municipal Transportation Agency and Muni have produced an unending stream of bad news,” said Supervisor Rafael Mandelman, who helped generate the idea after hearing the frustration of his constituents in the Castro, Glen Park, Diamond Heights and Noe Valley neighborhoods. He receives constant text messages and phone calls about late and overcrowded trains.

Other mayors have promised to overhaul public transportation — former Mayor Willie Brown said he’ll never live down his 1995 campaign pledge to fix Muni in 100 days — but Breed and her collaborators are taking the long view. Besides making the buses and trains run more efficiently, they’re also scrutinizing the leadership structure of the transportation agency.

After a year of meltdowns, officials in City Hall are floating reform proposals, including ideas to break the SFMTA into pieces and siphon its responsibilities, or give the district supervisors more control over a board that’s currently appointed by the mayor. The supervisors tried to do that in 2016 by pitching a charter amendment to voters, but it went down in defeat.

“This is a longer conversation,” Mandelman said. “Rather than racing to the ballot with a particular proposal, we’re going to look at what the MTA has achieved over two decades, where it’s failed, and what kind of funding it needs to be successful.”

Public transportation “requires constant renewal,” said former city Controller Ed Harrington, who has watched City Hall struggle to reform Muni for decades. It’s never easy to manage a complex system with a multigenerational stock of vehicles, and now officials are also grappling with a driver shortage and an economic boom that’s put more strain on mass transit.

Disruptions, such as the retrofit of the Twin Peaks Tunnel or the purchase of a new rail fleet, exacerbate these problems.

Harrington will co-chair the group along with SFMTA Board Director Gwyneth Borden. It will also include labor leaders representing transit workers, experts who have managed large systems in other parts of the country and transit advocates. Among them: James Gallagher, chief operations officer of the Los Angeles Metropolitan Transit Agency, Beverly Scott, former executive director of transit systems in Atlanta and Boston; and Mike Hursh, general manager of the AC Transit bus system in Alameda County.

“Muni has to work for everyone — for people commuting to work, for people getting to appointments, for people who are just trying to live their lives,” the mayor said in a statement.

That means the city must continue purchasing new trains and buses, Breed added. But it also could mean taking a scalpel to agency’s bureaucracy — or keeping it under watch.

Breed put the SFMTA on notice by sending a series of harshly worded letters this year, criticizing everything from sluggish bus service to the slow production of bike lanes to the seeming paralysis when riders complained. Separately, the Board of Directors began asking for monthly reports on Muni service, along with 90-day improvement plans from transit director Julie Kirschbaum.

“Monthly reports are fine and good, but this body is going to make expert recommendations about systemic reforms,” said Supervisor Aaron Peskin, who helped put the panel together. “They’re going to come up with suggestions that the decision-makers and ultimately the voters of San Francisco will have to grapple with, and answer.”

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