Under the plan being assembled, construction would still start in the Central Valley, but the upgrades to Caltrain and Southern California's Metrolink system would take place simultaneously.

Caltrain officials have longed to electrify their railroad for decades, and have completed plans but lacked funding. Electrification would allow Caltrain to run lighter, faster and cleaner trains, which officials believe would boost ridership. Along with an advanced train control system, which has been mandated by federal rail officials for commuter lines, it would also lay the infrastructure needed to carry high-speed trains up and down the Peninsula without significant construction.

Preliminary results of a Caltrain study show that an electrified railroad could accommodate two high-speed trains an hour -- at 110 mph -- without building additional tracks.

Caltrain would likely use part of the money to eliminate some of the Peninsula's 43 at-grade rail crossings by taking intersecting streets over or under the tracks, but building fully separated tracks, as on the BART system, won't be required, and could take place over time.

Murphy said Caltrain will also need to replace some bridges, tracks and rail ties to handle electrified and high-speed trains, and some of those improvements could also be included in a list of projects proposed to be funded by bonds.

Missing from the proposal is an extension to the Transbay Terminal. It's still in the plans, said Randy Rentschler, spokesman for the Metropolitan Transportation Commission, the Bay Area's regional transportation planning agency, but with a $4.2 billion price tag, it would be too costly to fund along with electrification.

"We can include a lot of things, but it can't be everything," he said. "We're trying to find the things that have the most impact."

The commission and the Bay Area Council, a business group that helped rally transportation officials to push for the accelerated Caltrain funds, are working to get agencies to assemble their list of projects and forge a funding plan that will require compromises and shifting funding from other projects. It needs to be done within weeks.

"It's not a done deal for the Bay Area yet," said Rufus Jefris, a spokesman for the council. "We need to come together on this."