The entire 800-mile line is scheduled for completion by 2033. There is no shortage of obstacles to what even the project’s biggest boosters call an ambitious timetable, including the engineering challenge of tunneling through the Tehachapi Mountains, a barrier between the Central Valley and Los Angeles.

The cost was originally supposed to be split among the state, the federal government and private business. But that arrangement faltered, as hopes for federal dollars faded with Republicans in power in Washington and businesses shied away from such an uncertain venture. As of now, the rail authority has come up with less than $30 billion of the necessary $100 billion, and the project’s costs are expected to continue to rise.

“The rest has to be found,” said Martin Wachs, an emeritus engineering professor at the University of California at Berkeley and a member of a committee appointed by the Legislature to review the project. “At the moment, 100 percent of the cost is going to be absorbed by the taxpayers.”

Beginning construction without all of the financing in place represents a strategic gamble by the rail authority, and by Mr. Brown, that once enough work is completed, future leaders will be loath to walk away from the project and leave a landscape of unfinished pillars, viaducts, bridges and track beds. Faced with reduced resources, the authority has altered its plans, and is now focused on finishing a 119-mile stretch of track from Bakersfield to Madera by 2022.

Brian P. Kelly, the head of the authority, argued that when this stretch of train is completed, people will rally around the project and the business community would become convinced of its viability.

“If I can get trains on the ground, Californians will start to see that this is something that we want,” he said. “There’s a lot of attention paid to what we don’t have. But we have significant fund-raising to get done what we have to get done.”

“Yes, the project has challenges,” Mr. Kelly said. “And the primary challenge for this same project is the same today as it was the day the voters passed the bond: And that is, we don’t have enough money to build what we want to build.”