The best route for the north-central portion of Calgary's next major LRT expansion would be to tunnel from 20th Avenue North, beneath the Bow River and all the way under downtown, finally emerging in the Beltline, according to the first-stage evaluation of the mega-project.

With an initial ballpark estimate of $1.8 billion, that was the most expensive option being considered by the city, but also the one that is believed to offer the fastest transit times and create the least disruption to existing traffic patterns.

The city is also looking at the possibility of building a new bridge to carry LRT cars over the Bow River, then either carrying the line above grade over downtown or building a shorter tunnel below the city centre. Those options were believed to be roughly $500 million less expensive than the full-tunnel option.

The cheapest option under consideration would be to run the LRT line down the middle of the existing Centre Street Bridge, pegged at about $700 million less than the full-tunnel route.

The full-tunnel route would see the LRT line run underground from 20 Avenue North all the way south to the Beltline in a tunnel beneath both the Bow River and downtown Calgary. (City of Calgary/Screenshot)

Those cost estimates are from February 2015, however, and they are only rough figures of a project that remains a long way from even getting started.

Monday's announcement that the city is leaning toward the full-tunnel option is just a "first step in designing the downtown segment of the Green Line LRT," according to project manager Jon Lea.

"We're starting to get a sense of what this project could mean for the downtown, and we're looking forward to continuing our work with Calgarians to further shape this new transit line," he said in a release.

In total, the city examined five possible routes through downtown, with various combinations of elevated, street level, and underground routes for the Green Line.

An evaluation of environmental sustainability, technical feasibility, cost, alignment with existing city policies, urban development, and public input led city staff to conclude "Option D" — full-tunnel route — best met the criteria.

"We hear often with the LRTs we have today: Why didn't you put them underground?" said Coun. Shane Keating.

"It is the most expensive option, but today is not really about the cost, trying to nail it down. It's saying 'What is the best for the City of Calgary?'"

Beltline route next to be evaluated

The Green Line's route through the Beltline is still to be determined and could include underground, street-level or elevated routes on 10th Avenue, 11th Avenue or 12th Avenue South.

Public input will be sought and more information can be found online and at an "interactive installation" the city plans to host at Olympic Plaza April 18 to 28, in addition to a series of "pop-up events" over the next two weeks.

The route through the Beltline would connect to the southeast portion of the Green Line, which is further along in the planning process, with the route already determined.

The goal of the multi-billion project is to have light-rail transit running all the way from the city's northern edge to the deep-southeast community of Seton.