The California High-Speed Rail Authority has released the Draft Environmental Impact Report/Environmental Impact Statement for the Palmdale to Burbank Project. The report identifies the route SR14A as the preferred route. What makes this interesting is, SR14A also happens to be the one with the longest bored tunnel, among the all alternatives.

The route SR14A includes 28 linear miles of bored tunnels out of total length of 38 linear miles. The route also has 1 linear mile long 'At-grade covered tunnel', 1 linear mile 'Cut-and-Cover tunnel', as well as 7 linear miles of At-grade profile and 1 linear mile of elevated profile.

To quote the report on the SR14A route:

"The SR14A Build Alternative alignment (Figure S-7) would begin in the city of Palmdale near Spruce Court on the west side of Sierra Highway. South of East Avenue S, the Refined SR14 Build Alternative alignment would curve eastward and south approximately 300 feet east of Una Lake. South of Una Lake, the SR14A Build Alternative alignment would curve westward, cross over the Metrolink Antelope Valley Line, Sierra Highway, and the Soledad Siphon, and continue southwest, entering a tunnel portal approximately 0.5 mile northeast of the Sierra Highway/Pearblossom Highway intersection. The SR14A Build Alternative alignment would then continue westward in an approximately 13-mile tunnel before surfacing approximately 0.75 mile east of Agua Dulce Canyon Road. The alignment would transition between at-grade and elevated profiles closely paralleling SR 14 before entering an approximately 1-mile tunnel. Transitioning from tunnel to at grade, the SR14A Build Alternative alignment would converge with the Refined SR14 Build Alternative alignment at the Vulcan Mine site. The remaining SR14A Build Alternative alignment south of the Vulcan Mine site would be identical to the Refined SR14 Build Alternative alignment"

Source: I drop a link to the summary of the report below. It is a PDF file (If you like, you can access the same document from the Authority's project website: Project Section Environmental Documents PALMDALE TO BURBANK)

Summary of the Draft Environmental Impact Report/Environmental Impact Statement for the Palmdale to Burbank Project