Here are a few of the challenges:
Cost estimates vary widely for boring two tunnels, each of which would contain two tracks. But costs can exceed thousands of dollars per square foot, Victor Romero, principal for Jacobs Associates in San Francisco, a civil-engineering consulting firm, said of tunneling generally.
One of the most expensive fixed costs is the boring machine, Beecham said, noting that it is so difficult to remove from the ground that generally the huge machine is left in the tunnel following the job.
A number of factors influence the cost, Romero said. A tunnel in an urban environment would have much higher costs than suburban tunneling. Soil conditions determine degree of difficulty and therefore costs.
On the other hand, the longer the tunnel, the more economy-of-scale is a factor, he said of fixed-costs involved.
Romero's firm — whose work includes the seismic upgrade of the Claremont Tunnel in Berkeley and design of the to-be-constructed fourth Caldecott Tunnel in Oakland <0x2014 > prices jobs by each element: labor, equipment, materials and other identifiable costs, plus contingencies.
"We never estimate per mile," he said, emphasizing that the cost of two different projects of similar length could vary greatly.
As for the length of time it takes to construct a tunnel, that, too, varies, Romero said. But it follows general stages: preparation of the site, including protecting adjacent buildings, which could take a half a year; the boring itself, including putting in the tunnel's structural lining, which could take up to a year or more; installing tracks, safety systems, ventilation and signals, which could take an additional couple of years; and finally building the station, which comes with its own challenges.
"The station takes longer to build than the tunnel," Romero said.
Menlo Park and Atherton have both joined environmental groups in suing the California High Speed Rail Authority over its environmental study, completed in May.
Local officials have protested the potential impact on businesses and residents if the route runs through the Peninsula, as presently planned.
Legal action could derail high-speed train plans, or at least keep them tied up in the courts for years.
On the other hand, the idea of tunneling could also sway some high-speed-rail opponents.
"It's pretty hypothetical, but certainly, putting all the tracks in a tunnel ... and being able to use the right of way for bike lanes and parkland would definitely bear a serious look," Menlo Park Councilwoman Kelly Fergusson, a civil engineer, said of the concept.
"It would have to be dramatically dense development" on the land to pay for the project, though, she said.
And there would need to be sufficient trust among all of the agencies. The worst-case scenario would be if the Rail Authority were to agree to the undergrounding then revert to above-ground plans if funding proved insufficient, she said.
Diesel in tunnels
Putting electricity-powered passenger rail lines in a tunnel is one thing; running diesel-fueled freight trains through is another.
The problem: fumes.
In addition to Caltrain's passenger service, Union Pacific sends about five to seven freight trains through the corridor a day, according to Zoe Richmond, the director of corporate relations and media for Union Pacific.
If the idea of undergrounding the railroad is to eliminate the current ground-level tracks, Union Pacific would have to operate in the tunnel, too.
Romero, of Jacobs Associates, noted that freight trains generally don't run in underground tunnels except for short distances.
"You see a bit of it on the East Coast, the Capitol Corridor," he said. "We tend not to run diesel underground."
But Carrasco is hopeful that it might be possible to install a ventilation system that would suck the fumes out of the tunnel.
Richmond said Union Pacific has trains that run through tunnels in the Sierra Nevada on the Sacramento-to-Reno stretch. Holes have been drilled in the mountainous terrain for ventilation. But engine operators also carry breathing apparatus, akin to what scuba divers use, in case of emergency.
Electrifying the freight trains is not yet feasible, Richmond said.
"That's definitely a challenge. So far the [electrification] technology isn't viable when it comes to freight trains," she said. Freight trains are heavier and require more power. In addition, she said, Union Pacific would need to electrify its whole system — a massive undertaking.
"The challenge is you can't just pick one area to electrify," she said.