Early route maps show the stations five blocks apart, or three blocks as the crow flies. Businesses, politicians and transit advocate say a single station couldn’t handle future ridership demand in the booming neighborhood.

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At first glance, the plan looks like a no-brainer.

Sound Transit has promised a pair of subway stations for South Lake Union, one of the nation’s fastest-growing neighborhoods and home to thousands of transit-riding workers.

But early route maps show these stops a mere five blocks apart, or three blocks as the crow flies, because of a bend in the $6 billion light-rail corridor from downtown to Ballard.

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And they’re expensive. The agency estimates the cost of a second SLU stop at over a half-billion dollars — to create a slightly larger area where people can easily walk to a train. Construction is to be done by 2035, in the ST3 plan voters passed in 2016.

Meanwhile, money is tightening as community groups seek unbudgeted improvements, such as Ballard and West Seattle tunnels that may require hundreds of millions of dollars.

Dual SLU stations enjoy unanimous support by businesses, politicians and transit advocates, who compare demand here to New York or Washington, D.C. Barring a fiscal collapse, both stations will enter the preliminary design phase next year.

“I think we need to spend the money,” said Mike McQuaid, transportation committee chairman of the South Lake Union Community Council. “I think we need to invest in the future of this city.”

But how much is too much, to give thousands of train customers a shorter walk?

“Hopefully, somebody’s asking the question and looking at the options, because these are big decisions,” said Steven Polzin, director of mobility research at the University of South Florida. “Are there ways of moving people in lieu of a relatively expensive investment?”

Aurora station

Three years ago, a single SLU station at the corner of Westlake Avenue and Denny Way appeared in the map issued by consultant Parsons Brinckerhoff, under its $10 million planning contract.

Community groups requested more, and then-Seattle transportation director Scott Kubly wrote a letter in mid-2015 supporting another stop where Aurora Avenue North meets Harrison Street.

Sound Transit continued to call Aurora a “potential additional station,” before adding it into ST3 in early 2016. Estimated costs were $380 million in 2014 dollars, or $544 million when construction begins.

It was a low-profile move.

“I don’t recall having specific discussions,” said University of Washington professor Mark Hallenbeck, chairman of the state-appointed ST3 Expert Review Panel. The panel focused on more basic questions, he said, like whether to tunnel under Queen Anne Hill into Fremont.

Sound Transit’s 25-year finance plan shows enough tax income to build both SLU stations, but not other features people are demanding.

A Ballard coalition wants a Salmon Bay tunnel instead of the proposed 70-foot-high drawbridge that would open two to four times a day.

The West Seattle Junction Neighborhood Organization suggests a half-mile tunnel topped by a small park, while canceling the elevated Avalon Station downhill.

Seattle Transit Blog supports increasing the Aurora station to two track levels, which it calls a “future proof” layout that allows a tunnel one day to Fremont and Green Lake.

“There’s absolutely no reason to think we have to sacrifice anything. We can have everything,” said Drew Johnson, volunteer coordinator for the nonprofit Seattle Subway.

Seattle is one of the richest cities in the world’s richest nation, and a capital-gains tax or other progressive sources are available, Johnson said.

For now, Seattle-area politicians are entertaining more ideas, and not choosing which features should take priority.

“In general, the costs and benefits should be weighed against other opportunities,” said Carole Turley Voulgaris, professor of transportation engineering at Cal Poly, San Luis Obispo

“It could be extending a line somewhere else. It could be improving service. It’s not so much a question of does it pass or fail. It’s a question of, what’s the best kind of investment?” Voulgaris said.

Ridership estimates grow

If anything, the case for building the twin stations has grown since the 2016 election.

Initially, a Sound Transit fact sheet said the additional Aurora stop would serve 3,000 to 4,000 daily boardings.

Those numbers, said Hallenbeck, ignored a cultural shift toward transit now that roads and highways are maxing out. Rail is the way to keep pace with job growth, he said.

“Unless Amazon goes bust, they’re going to underestimate ridership,” Hallenbeck said.

In response to Seattle Times questions, Sound Transit reran its model — upping its forecast to 7,300 to 9,400 daily boardings at Aurora, and 13,000 to 17,000 for Denny, spokeswoman Kimberly Reason said. The agency wouldn’t provide supporting documents.

By comparison, at Westlake Station some 12,700 people board trains each weekday and another 6,100 hop a bus.

That SLU is becoming like downtown is a good reason to err on the side of more capacity, said Yonah Freemark, author of the national Transport Politic blog, which tracks ridership and federal policies.

“Depending on how successful the first station is, it would be useful to have an additional station to deal with overflow,” he said. “You only get so many chances to build a tunnel through downtown.”

Generally, a station adds 42 seconds to a train trip. But if you build too few stations, and they’re packed, any time saved would be lost as crowds jostle their way into the trains.

If a boarding platform at Aurora does serve 8,000 riders a day, that is “pretty darn busy,” Polzin said.

By comparison, central stations in Toronto each handle 23,000 to 56,000 daily boardings. Washington, D.C., stops range from 14,000 to 25,000. Vancouver hovers near 21,000 each.

So if Sound Transit were to build a single, consolidated SLU station, the foot traffic inside might resemble the busiest stations in peer cities.

Bus rider David Beatty, after seeing maps at a Ballard Sound Transit forum, called the dual-station plan “duplicative.” He favors a combined stop near Aurora. If tax dollars turn tight, he’d rather see a Salmon Bay tunnel, or even a second Ballard station farther north.

“I’d think the Denny Station would not have a lot of defenders, if you do something great in Ballard,” he said.

Underground stations

Property to build stations is vanishing. If any did exist, planners estimated $74 million just for Aurora-area land.

So Sound Transit assumes both SLU stations must be built under the street, said Cathal Ridge, the agency’s central corridor director.

These require an excavation 400 feet long to handle a four-car train plus exit and maintenance areas. And they’ll be about 90 feet deep, so the tracks can dive under both the existing Westlake Station and the new Highway 99 tunnel.

Station construction will block more road lanes and sidewalks than earlier light-rail digs across town, where trucks could park off-road to remove dirt and deliver concrete.

“It’s going to be more like when the initial Downtown Seattle Transit Tunnel was built in an urban, dense area, than a Roosevelt or UW or Capitol Hill (station),” said Don Billen, capital project development director.

The open-trench tunnel segment of that late 1980s project closed Pine Street. The section under Third Avenue was mostly bored below ground, but stations required surface digs and lane closures.

The American Public Transit Association recommends urban stations 0.3 to 1.3 miles apart, depending on population or job density.

The dual SLU stations are proposed at 0.4 miles apart, or slightly closer than in downtown Washington, D.C., on average. Supporters compare SLU to the Westlake and University Street stations downtown, also five blocks apart. From there it’s eight blocks to Pioneer Square Station, and eight more to International District/Chinatown Station.

The Mercer Stakeholders coalition, which includes big South Lake Union employers, argued in a letter to Sound Transit that a combined SLU stop would sit 0.7 to 0.8 miles from either an Uptown or Westlake station — “too far apart to effectively serve these dense urban areas.”

Seattle DOT wanted the Aurora station, Kubly said, because people could hop off the RapidRide E bus there and walk down to meet a train.

Some citizens suggested a broader westward curve, placing the second SLU stop farther north along Mercer Street. This would shorten the walk to UW Medicine, Allen Institute for Brain Science and Seattle Cancer Care Alliance. But the city would have to relinquish its dream of a Harrison-Aurora transit hub.

Another option that Ridge says will be examined is moving the Aurora stop west, alongside the Bill & Melinda Gates Foundation. But land might not be available there.

Seattle resembles many U.S. regions where local politics drive infrastructure costs higher than spartan 20th-century subways or international norms, said Polzin, the Florida mobility expert.

“We’ve got lots of stakeholders and communities. To keep everybody happy, everybody gets everything they want, and the projects become more expensive,” he said. “There is no discipline at this point.”

Chances are, it takes a recession or catastrophic cost overruns for politicians to backtrack on any campaign promise, Polzin said.

Sound Transit did so in 2005, when feared cost overruns for a deep First Hill Station drove the board to cancel it. Overruns of 86 percent systemwide delayed the 2006 opening of a University District station until 2021.

No day of reckoning appears now.

“We told voters we’re going to be building these two stations,” said Seattle City Councilmember Rob Johnson, who is on the transit board. “I think it’s incumbent upon us to build the stations the voters approved.”

A full 70 percent of Seattle ballots supported the ST3 plan.

Correction: An earlier version of this story misstated Mike McQuaid’s current position with the South Lake Union Community Council. He is transportation committee chairman.