But the state used a lower cost estimate when it issued its 2014 business plan four months later.

A confidential 2013 report by the state's main project management contractor, New York-based Parsons Brinckerhoff, estimated that the cost of building the first phase from Burbank to Merced had risen 31% to $40 billion.

And it projected that the cost of the entire project would rise at least 5%.

Parsons Brinckerhoff briefed state officials on the estimate in October 2013, according to the document obtained by The Times.

Doing so will meet a commitment to begin carrying passengers between Burbank and Merced in the first phase of the $68-billion high-speed rail link between Los Angeles and San Francisco.

However, a Times analysis of project documents, as well as interviews with scientists, engineers and construction experts, indicates that the deadline and budget targets will almost certainly be missed — and that the state has underestimated the challenges ahead, particularly completing the tunneling on time."It doesn't strike me as realistic," said James Monsees, one of the world's top tunneling experts and an author of the federal manual on highway tunneling.

"Faults are notorious for causing trouble."The California High-Speed Rail Authority hasn't yet chosen an exact route through the mountains.

It also is behind schedule on land acquisition, financing and permit approvals, among other crucial tasks, and is facing multiple lawsuits.

The monumental task of building California's bullet train will require punching 36 miles of tunnels through the geologically complex mountains north of Los Angeles.

Crews will have to cross the tectonic boundary that separates the North American and Pacific plates, boring through a jumble of fractured rock formations and a maze of earthquake faults, some of which are not mapped.

It will be the most ambitious tunneling project in the nation's history.

State officials say the tunnels will be finished by 2022 — along with 300 miles of track, dozens of bridges or viaducts, high-voltage electrical systems, a maintenance plant and as many as six stations.