They're apparently still trying to build that California High-Speed Rail, and a proposal to make it tunnel under the San Gabriel Mountains—first floated last summer—is getting serious. (The hope is that it'll be more popular with people who weren't so keen with the dominant plan to have the train travel along Highway 14 through Acton and Santa Clarita.) According to the California High-Speed Rail Blog, there are four options for the train's path from Burbank to Palmdale and three of them involve long tunnels under various sections of the mountains. The details of each proposed bullet train path are laid out clearly in these videos from the high-speed rail authority, which give all four the animated fly-over treatment—very helpful when trying to figure out exactly where this nearly $68-billion bullet train could end up traveling.
The Highway 14 alignment (the default option)
First East Corridor Route (E1): Of the three East Corridor routes, this is the westernmost option. Notable features include "a tunnel that branches off the San Fernando Road corridor near Whiteman Airport and goes under Pacoima as well as the mountains, emerging along the Metrolink tracks near Soledad Canyon."
Second East Corridor Route (E2): A tunnel under Burbank Airport that becomes a bridge across the Tujunga Wash that goes back to being a tunnel again until it hits Soledad Canyon.
Third East Corridor (E3): The easternmost option, this involves a tunnel from Burbank Airport to the Antelope Valley north of the Vincent Grade.
· CHSRA Releases New Animations of Palmdale-Burbank Routes [CHSPB]
· LA Bullet Train Could Run in Tunnel Under the San Gabriels [Curbed LA]