The spiffy new Transbay transit center will have to add Caltrain and high-speed-rail service to become more than a $2.2 billion bus terminal, but transportation officials believe new leadership is needed — along with a sexy new name and image — to win funding and complete construction of the San Francisco transit hub’s second phase.

A collection of agencies is scrambling to put together an agreement that will help reboot what’s now referred to as the Caltrain downtown extension, or DTX for short. They’re hoping to complete the process and start the rebranding process early next year, if not sooner.

“Maybe that’s an opportunity to get some excitement going and get some ideas,” Boris Lipkin, the Northern California director for the California High-Speed Rail Authority, said of the idea to rename the project.

“Hopefully, we don’t get too many Tunnel McTunnel Face suggestions,” he added, referring to a popular meme.

The fresh start for the long-envisioned rail link was recommended by a panel of experts on large infrastructure projects in a recent study for the San Francisco County Transportation Agency. The agency withheld $9.6 million in sales-tax funding for the downtown extension in 2018 while it studied whether the project needed an entity other than the Transbay Joint Powers Authority to oversee the project’s construction.

The experts recommended the joint powers authority stick to only operating the Transbay transit center as talks continue between Caltrain, the city of San Francisco, the California High-Speed Rail Authority, the San Francisco County Transportation Authority and the Metropolitan Transportation Commission on creating an executive steering committee that would run the DTX project. The joint powers authority would oversee this work.

Various routes or extensions have been suggested, but the project that will be built is a 1.3-mile subterranean rail connection that stretches from Caltrain’s current San Francisco terminal at Fourth and King streets beneath Townsend and Second streets, where a now-vacant train box sits in the basement of the transit center.

Other ideas, including a tunnel starting on Pennsylvania Avenue, or a connection to a second transbay train tunnel, could be considered later, officials said. The hub will retain the current ground-level terminal and add an underground station at Fourth and Townsend streets.

The last cost estimate for the extension — issued in 2016 — was $4 billion, but most officials think the current cost could be closer to $6 billion. Mark Zabaneh, executive director of the joint powers authority, said the project has about $2.5 billion in the bank.

Once an agreement is reached on who’s running the project, money will once again flow from the city’s transportation sales tax, said Tilly Chang, executive director of the Transportation Authority. But the project will still need billions of dollars in state and federal funding.

To get that kind of money, the experts suggested in their report, the downtown extension should get a new name that projects an image of regional, statewide or even national significance.

“This project is bigger than San Francisco,” said Supervisor Aaron Peskin, chairman of the Transportation Authority. “It has to be seen as one of most importation projects in the state of California.”

Asked to suggest a name more likely to attract attention — and funding — Peskin paused before saying, “San Francisco-Silicon Valley Express.”

The idea of bringing Peninsula commuter trains downtown has been discussed since the early 1900s. Plans for the extension to the Transbay transit center began in 1999. Now, with the Transbay transit center open following a 10-month closure due to broken steel girders, the focus has turned to bringing rail to the center.

“It’s a beautiful facility, but it’s not really complete until a train shows up,” said Randy Rentschler, legislative director of the Metropolitan Transportation Commission, the Bay Area’s transportation financing and planning agency.

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