SACRAMENTO — Plans for a high-speed train between San Francisco and Los Angeles face an uncertain future after Gov. Gavin Newsom said Tuesday that he would focus on building only the Central Valley segment of the rail line.

Newsom, in his first State of the State address, also said he would push for just a single tunnel under the Sacramento-San Joaquin River Delta, rather than the twin tunnels that former Gov. Jerry Brown proposed to ship water to Central Valley farms and Southern California.

The changes amounted to a significant break by Newsom on two of Brown’s legacy projects, both of which have come under financial and regulatory challenges.

“We face hard decisions that are coming due,” Newsom said during a speech that served as a reality check on the more soaring rhetoric he offered during his inauguration last month. “The choices we make will shape our future, and the future of quite literally millions, for decades to come.”

A bullet train that would speed people between San Francisco and Los Angeles nearly as fast as they could travel by plane was first conceived under former Gov. Arnold Schwarzenegger. But Brown enthusiastically endorsed it throughout his second two-term stint as governor, even as it missed construction deadlines and costs soared.

Newsom poured cold water on the prospect it would ever be completed as his predecessors envisioned, though he didn’t completely abandon the idea.

“There simply isn’t a path to get from Sacramento to San Diego, let alone from San Francisco to L.A.,” Newsom told a joint session of the Legislature. “I wish there were.”

He said the current project would “cost too much and, respectfully, take too long.”

But a link between Merced and Bakersfield, he added, could serve as a revitalizing force for the Central Valley, a region Newsom has promised not to neglect.

“I know that some critics are going to say, ‘Well, that’s a train to nowhere.’ But I think that’s wrong and I think that’s offensive,” Newsom said. “It’s about economic transformation. It’s about unlocking the enormous potential of the valley.”

California voters approved a $10 billion bond in 2008, largely for the construction of the San Francisco-to-Los Angeles bullet train. The state finally broke ground in 2015 on a 119-mile segment between Madera and Bakersfield, but the train has lost popularity and become a growing political target as it blows through deadlines and budgets.

A revised business plan adopted last year pushed back the completion of the project by an additional four years, to 2033, and estimated a final cost of $77 billion, more than double the original price tag.

Central Valley Republicans, who have been among the most vociferous critics of high-speed rail, celebrated what they interpreted as the impending death of the project. House Minority Leader Kevin McCarthy of Bakersfield said in a statement that high-speed rail is “finally arriving at its deserved destination — extinction.”

McCarthy, a close ally of President Trump, is an obstacle to securing further federal funding that the state would need to finish construction of the train line to San Francisco and Los Angeles. He said he looked forward to working with Newsom and federal officials to “best mitigate what has already been wasted on high-speed rail due to the previous administration.”

But Democratic legislators, many of them from the Bay Area and Los Angeles County, said they supported Newsom’s plan as a first step to eventual completion of the entire line. Assembly Speaker Anthony Rendon, D-Paramount (Los Angeles County), said it would be “problematic” if the bullet train did not ultimately link the state’s two biggest urban areas.

“I’m optimistic that we want to see this move forward, but in a way that is responsible and respects what we told the voters we would do,” said Senate President Toni Atkins, D-San Diego.

Newsom promised to continue the environmental work needed for the Bay Area and Los Angeles sections of high-speed rail and to push for more federal and private funding. He also said he would appoint a new chair of the High-Speed Rail Authority, his economic development director Lenny Mendonca, and bring more transparency to the project by posting spending online.

“But let’s get something done, once and for all,” he said.

After previously toying with the idea, Newsom also committed Tuesday to a preference for one delta tunnel, rather than the twin tunnels that Brown wanted to build to ensure more reliable water delivery to farmers and Southern California.

Newsom said he would appoint a new chair of the California State Water Resources Control Board, Joaquin Esquivel, whose first task would be to reach an agreement on the water project. It has drawn strong opposition from farmers and environmentalists in the northern part of the state, who worry that too much of their water will be sent south and that diverting water from the delta will harm its overall health.

“We have got to get past these old binaries, like farmers versus environmentalists, or north versus south,” Newsom said. “Our approach can’t be ‘either-or.’ It has to be ‘yes-and.’ Conveyance and efficiency.”

Alexei Koseff is a San Francisco Chronicle staff writer. Email: alexei.koseff@sfchronicle.com Twitter: @akoseff