The LA Weekly covers talk about a train tunnel under the Sepulveda Pass:
Measure R, the half-cent sales tax approved in 2008, provides $1 billion for a Sepulveda Pass transit project. But the Sepulveda Pass is so steep that the only way to put a train through it is by building a six-mile tunnel. Current estimates put the cost of such a project at $4.8 billion to $5.5 billion.
Though Mayor Antonio Villaraigosa endorsed the train idea back in April, the Metropolitan Transportation Authority has not taken a position on the project. Other possibilities include creating a bus lane on the shoulder of the 405 freeway, turning carpool lanes on the 405 into toll lanes, or building an aerial bus-and-toll lane.