Just because trains are running and developers are developing doesn’t make the Green Line complete. Unless high-quality green space is added, the $957 million transit project connecting the downtowns of Minneapolis and St. Paul can’t live up to full potential or produce a maximum return on investment.

“It will have been a missed opportunity,” said Jenna Fletcher, local program director for the Trust for Public Land, which last week released an insightful report urging a significant public/private campaign to plan, build and maintain a string of parks, plazas and linear green spaces along the corridor. It’s an idea worth supporting.

The report, “Greening the Green Line,” offers no cost estimates or timelines. But it correctly diagnoses the problem and acknowledges the challenges. It also lays out the strategies needed to transform the largely bleak landscape in ways that would enhance the lives of those living and working nearby while also drawing newcomers to the heart of the metro region. Indeed, combining sustainable transportation, housing, jobs and green space has become the competitive model that nearly all successful cities now follow.

Using green space to boost livability and economic value is hardly new to the Twin Cities. Minneapolis’ decision in the 1880s to preserve miles of lakeshore for the public was pivotal to linking the urban and the pastoral, thus satisfying Minnesotans’ uncommonly strong desire for natural surroundings, even in the city. It also demonstrated the value of green space as an economic catalyst in residential neighborhoods. Parks boost the value of nearby homes by as much as 20 percent.

But green space was, for the most part, left out of the downtowns and other commercial or industrial districts. And those are the places (North Loop, Downtown East, Midway, Lowertown and others) now being converted to walkable neighborhoods with housing, jobs, retail and transit. Unfortunately, green space has been an afterthought, and, in any case, city park budgets are stretched to their limits. That’s why creative public/private models are needed to meet the growing demand.

One key is to recognize the changing market driven by aging boomers needing walkable spaces, by millennials wanting an authentic but green urban experience, and by disadvantaged populations hoping for safe places for families and children to gather. Another key is to accept the changing definition of parks.

In an urban setting, parks are not just turf, trees and trails, but also plazas, linear streetscapes and rain gardens that double as stormwater-management systems. Even surface parking lots can be pleasant community assets if trees, permeable pavers, iron fencing and pop-up programming are added.

Better public spaces also must adapt to Minnesota winters, not just by adding cold-weather programming but by removing snow, especially along curbs where windrows of blackened slush and ice can create ugly barriers that hinder pedestrians and give off an impression of public negligence.

Paying for all of that requires new kinds of governance models to complement traditional park agencies — for instance, corporate adopt-a-park programs; public/private conservancies like the one now being formed in downtown Minneapolis; garden club initiatives like those that help sustain St. Paul’s iconic Rice and Mears parks, and incentives for developers to set aside public green space as part of their projects.

“It’s not a matter of ‘either/or’ but ‘both/and,’ ” said Fletcher, adding that petty rivalries on the local parks scene must be replaced by a willingness to innovate and collaborate.

The University of Minnesota’s main campus, near the center of the Green Line corridor, should stand as an inspiration. Fifteen years ago, the U’s grounds were an embarrassment. To remain competitive — to draw students and faculty — then-President Mark Yudof recognized the need for a green makeover. Now the U has one of the prettiest urban campuses in the nation. For similar competitive reasons, the Green Line should lay the groundwork for turning truly green.