Portland, Oregon, transportation planners presented their vision for long-range regional transit improvements last week to the Portland Planning and Sustainability Commission. Tucked among descriptions of improved bus service, light-rail extensions and streetcar enhancements, there’s a map with a dotted purple line curving in an S from downtown across the Willamette River. The line is labeled Potential Subway.

“The point of long-range planning like this is you’re never done building a city,” says Dylan Rivera, Portland Bureau of Transportation spokesman. “We’re going to continue to grow for the foreseeable future, and we might need to look at some really aspirational big ideas. A potential subway would be in that category.”

Metro, the Portland region’s metropolitan planning organization, is working on its 2018 update to the 25-year transportation plan. The idea of digging a tunnel underneath downtown for high-capacity rail has been floated on and off since the ’90s. As regional agencies look ahead and think about the capacity of existing surface transportation, the subway is back on the table.

If it happens, it won’t be anytime soon, however.

“We think it’s probably in the 30- to 40-year time frame,” Rivera says. “There’s certainly no budget or project details yet. Obviously building underground is extremely expensive and would require a lot of study before we’d consider it further. But we think it’s worth keeping in mind as we pursue more short-term and mid-term projects.”

The idea, theoretical as it is, has gained support among some transit advocates.

“We should absolutely look at all options in terms of investing in transit, and we should think boldly about increasing transit capacity,” says Stephanie Noll, interim executive director of The Street Trust, a nonprofit that works to promote transit and a more walkable and bikeable Oregon. “Our road infrastructure simply will not be able to accommodate a growing population if people keep relying on personal vehicles as they are today to travel around.”

That said, even the hypothetical possibility of spending billions on a subway line gives Noll pause. “It’s a lot cheaper to dedicate existing travel lanes [for bus rapid transit] on our road network, but often that’s not politically popular,” she says.

Portland is going to have that fight whether the subway happens or not. Bus-only lanes are part of the city’s transit improvement plans.

“In the near term, we see a lot of need to invest in our bus system and to do as much as we can to get buses out of congestion,” says Rivera.

For PBOT that means adding transit signal priority, queue jumps and bus-only lanes, especially near bridge bottlenecks. TriMet, the regional transit agency, has enhancement plans that include increasing bus and light rail frequency, improving connections to transit hubs, extending service hours, and more. On SE Division Street, TriMet is building a new, high-frequency bus line (whether it qualifies as bus rapid transit is up for debate). In southwest Portland, which currently lacks light-rail service, TriMet is working on a new line that could open by 2025.

Noll thinks the TriMet plans are promising, but has concerns about the potential uphill battle it will require. “I think we need to increase both public understanding and support for bus-only lanes,” she says. “That’s often a hard political battle. It’s easier to talk about frequency than it is to talk about reallocating space on our roadway.”

Beyond that, Noll thinks PBOT and TriMet need to be thinking hard about the pedestrian and bicycle connections to transit hubs. She says connections in the city center are pretty good already, but on the edges of the city and beyond, where some of the planned transit expansions will go, those connections are lacking. “There’s not a complete sidewalk network in our region yet,” Noll says. “Good pedestrian connections means completing sidewalks, making safe crossings, just creating a comfortable environment for people.”

Looking far into the future again, Rivera tells me the subway is not the only gigantic, aspirational, multibillion-dollar project PBOT planners are chatting about. There is also talk of burying Interstate 5 along the east side of the Willamette River to open the east riverbank for more housing and commercial development and replacing the interstate’s double-decker Marquam Bridge river crossing with a tunnel.

“That’s probably even further away than a transit tunnel, but it’s another favorite ambitious project that people like to talk about,” says Rivera.