Long before Palo Alto neighborhoods quivered at the thought of elevated high-speed trains whishing over their heads at 125 mph, a group of civic leaders and architects was hashing out its own vision for the city's future -- one that includes fast trains, leafy bike paths, dense developments, a hub and a really deep tunnel.

The vision -- advocated by local architect Tony Carrasco, former mayor Bern Beecham, Councilman John Barton and other prominent city leaders and architects -- would raise about $700 million by selling air-rights above the Caltrain corridor to developers and use proceeds to build a deep tunnel under the corridor for the new high-speed rail system.

The group has modified its plans since September as it learned more about train tracks dimensions and other technical details. Carrasco said the group also changed its plan for the 6-acre El Camino Park, which under the current version would house developments. The group proposes to compensate for the loss of green space by creating a 31-acre linear park stretching along the Caltrain Corridor.

The group also abandoned its initial idea to build townhouses along Alma Street. As detailed in current sketches, Alma would be a bicycle-friendly, tree-lined boulevard.

The vision, the group notes, isn't exactly a plan, but rather a conceptual exercise that seeks to promote further dialogue. No one expects it to be implemented in its exact form or in the near future. But the proposal has already piqued the interest of top urban designers and, last month, earned a prestigious accolade -- an Honor Award in urban design from the American Institute of Architects California Council.

The proposal, entitled "From an age of movement to an age of place" attempts to link Stanford University, downtown Palo Alto, Stanford Shopping Center and Stanford University Medical Center with a bustling "innovation place" -- a gathering area for friends, neighbors and networking entrepreneurs. It also imagines Alma Street as a tree-lined boulevard next to an underground rail system.

The trains, under the plan, would run below surface and would stop at a new station in downtown Palo Alto. Residents could mingle downtown, grab some dinner, run their errands and take the train to San Francisco, for example, to catch a ballgame.

"Our proposed high-speed rail system, rather than dividing Palo Alto, unites the community," proclaims a slide in the group's presentation.

But the new hub would come with a price: greater density. The plan would create a more urban environment around downtown by increasing density in retail, office and residential developments.

These developments, the plan states, would be "designed in character with the city" and consistent with city policies that encourage dense, mixed-use development near major transit centers.

Barton, who is also a principal in the architecture firm Barton Architect, said the goal of the exercise was to begin a conversation about what types of tradeoffs the city would have to make in the coming years. He said he doesn't expect everyone to embrace the details of the group's concept, but rather hopes other people would use it to develop their own proposals and ideas.

Barton said he was impressed by the passion and knowledge displayed by the Palo Alto community when it comes to the high-speed rail.

"A lot of folks are concerned and scared and angry at what it can be," Barton said. "But most people also say that, in general, the idea is good and we should work together to make sure it works for the city."

"The point of this exercise is to say, 'Here are the kind of tradeoffs we'll have to look at,'" Barton said.

Carrasco said the group will present its concept at an urban-design workshop the City of Palo Alto plans to host in early October. The workshop will bring noted architects, finance specialists and train experts to the city to discuss integrating the high-speed rail system into Palo Alto's urban setting.

In the meantime, Carrasco said the group is taking a closer look at its numbers and trying to determine, as accurately as possible, how much a train tunnel would cost and what kind of options Palo Alto has for financing it.

"This is only meant to stimulate discussion," Carrasco said. "We're hoping the community will turn out to help shape the proposal."