California's high-speed-rail project could offer Palo Alto and its neighboring cities a rare opportunity to revitalize their downtown districts, transform old train tracks into leafy gateways and bring neighborhoods closer together, a group of leading urban designers and architects said at a Sunday workshop.

The meeting, held at the Sheraton Hotel in Palo Alto and attended by about 80 persons, was the conclusion of a two-day workshop organized by a group of local architects and sponsored by the Peninsula Cities Coalition.

On Saturday, about 80 residents and experts broke into groups, each focusing on one mile of the Caltrain corridor between Atherton and South Palo Alto.

On Sunday, the eight maps were meshed into what event organizer Brian Steen referred to as an "opportunity map" -- the participants' wish list for the controversial $40 billion line that California voters approved in November, along with a $9 billion down payment.

The list included new parks, sculptures, open spaces, bike paths and walkways along the Caltrain corridor. It also included an enhanced business district, urban gardens and rebuilt civic centers.

"What we've seen in the last 48 hours is the creation of a vision for our communities -- a vision we all know is idealistic, but we feel parts of it are achievable," Steen said.

The various groups agreed that the portion of the high-speed rail zipping through the Peninsula should go through an underground tunnel, though there was no consensus whether this should be a cut-and-cover "trench" tunnel or a more expensive bored tunnel deeper underground.

Under the common vision, the Caltrain corridor would be converted into a bicycle and pedestrian corridor spanning eight miles from Atherton to South Palo Alto. As it passes through Palo Alto, the green path brings together neighborhoods that have long been separated by the tracks.

A new network of parks, plazas and dense developments crops up to link University Avenue with Stanford Shopping Center. The downtown area transforms into a green mingling district for Palo Alto residents, academics and entrepreneurs.

Tony Carrasco, a local architect who organized the weekend event, said the goal is to bring "connectivity" to Palo Alto's neighborhoods and residents. Carrasco is part of a group of architects and land-use experts who have long advocated for underground tunnels in place of the current train tracks.

"We need to go beyond ourselves and look at the next 100 years or 200 years and ask, 'What kind of an opportunity do we have out here?'" Carrasco said. "Do we want to be able to take an elevator to the ground floor and go to the grocery store and meet our friends? I think the answer is yes. Not for everyone, but for a large majority."

Judith Wasserman, a member of Palo Alto's Architectural Review Board, gave the plan a special name: "Together Again for the First Time." Wasserman said an underground system could offer the city a long-awaited chance to connect its neighborhoods.

"The town has always been divided by the train," Wasserman said. "We've never had good cross-town connection. This is an opportunity we'll never have again."

But speakers at Sunday's workshop also acknowledged a major obstacle standing between them and their idealistic vision: the high cost of creating underground tunnels. While rail officials don't expect to have a cost estimate for the project for another year, they have estimated the cost of boring tunnels to be about 6.5 times as much as building at grade.

The cost of building underground tunnels is also expected to be beyond the rail authority's $4.2 billion budget for the Peninsula segment.

Glenn Isaacson, principal at Conversion Management Association, said cities would have a hard time funding a tunnel, but offered several ways in which it could be done. Aside from passing bond measures or enacting special taxes, cities could sell land currently occupied by the Caltrain right-of-way and use the proceeds to pay for the tunnels, he said.

Workshop participants have also singled out a downtown stretch between El Camino Real and Alma Street as a possible site for dense, revenue-generating developments, including multi-story condominiums. But Isaacson warned that the project would still likely require significant additional funding from the California High-Speed Rail Authority.

"I'd urge you to watch your pennies in the cost of what you select," Isaacson advised the audience. "You'll have a hard time covering 100 percent of the gap."

Dominic Spaethling, regional manager with the rail authority, said the agency is in the process of holding workshops and narrowing down possible alternatives for the San Francisco-to-San Jose segment. Spaethling called the two-day workshop in Palo Alto an example of the "context sensitive solutions" method to rail design. The approach includes collaboration with stakeholders to minimize the impacts of major transportation projects.

"Whether you realize it or not, it's sort of what we're doing here -- realizing that the high-speed train is one element of a larger vision," Spaethling said.

"This is how it begins."