It's the most expensive and most complicated part of the biggest project Calgary has ever built.

And the head of the Green Line team knows they have to get it right.

On Tuesday, Michael Thompson revealed the revised plan for getting the city's next LRT line through the heart of the city.

The tunnel has been shortened and the number of underground stations has been reduced by one. It's all designed to help the project stay within its $4.9 billion budget.

CBC News asked Thompson for more details of the new alignment, moving from 16 Avenue N. to the Beltline.

This interview was edited for clarity and length.

CBC: There will be a level crossing on Centre Street at 16th Avenue N. Temporarily, that will also be the end of the line for the CTrain. So how do you do that in that intersection without creating a huge traffic snarl in all four directions?

Thompson: So the station will actually be just to the south of 16th Avenue. So we actually won't cross 16th Avenue and we'll have a station just to the south of 16th Avenue. So we're actually not crossing 16th Avenue at grade right now. We'll have a station to the south and the train, the project will start and go south from there.

CBC: Can you explain how a CTrain will get from running in the middle of Centre Street over to the new bridge that would carry it over the Bow River?

Michael Thompson is the general manager for the City of Calgary's Transportation Department. (City of Calgary)

Thompson: The Green Line is a low-floor LRT, so it's not the same type of LRT that we have right now. Lower stations, smaller vehicles — so it doesn't have the same physical impact on the environment around it. As we run down the middle of Centre Street, there are going to be no barriers between the train and the adjacent vehicles. So, think of a low-floor LRT like you would see in Europe or in cities all across North America. So it's not like 36th Street, where we have a concrete barrier between the traffic and the train. It's really integrated into the entire road cross-section. Then, as we go down from 16th Avenue to the south, we'll have a set of signals. The train will cross, go across the single lane of road traffic going southbound on Centre Street, and get onto the new bridge.

CBC: There's the escarpment there. There are houses up above. Are you going to cut into the escarpment or is it just a turn before the Centre Street bridge and away it will go?

Thompson: Those are details we're working through still to understand exactly what that will look like.

CBC: When that bridge over the Bow is constructed, can you put a pathway or a pedestrian bridge underneath it like the existing LRT bridge over in Sunnyside?

Thompson: It's something that we'd be looking at. Obviously, we've heard from a number of stakeholders already that they're interested in pedestrian and cycling connections on bridges like that. It's an important piece to be able to cross the river with bridges and for people to be able to cross walking and cycling from the north to the south. So we'll take a look at that.

CBC: How do you construct a cut-and-cover tunnel along 2nd Street S.W. that crosses every major east-west road downtown? All at once or in phases?

Thompson: The means and methods of the construction would be determined by the contractor. What we're doing is we've lifted up the elevation of the tunnel by not going underneath the river and crossing over top of the river. Our stations will be shallower, but we leave it to the contractors who really know their trade about how they would go about constructing that tunnel, what means and methods they would use. There's a few different types they would use, depending on the specific geology and the depth that they ultimately design it to be. And so that's something they'll determine. What we've determined is that it's possible to lift it up and that we can reduce our costs and reduce our risk.

The new downtown Green Line tunnel has been shortened and the number of underground stations has been reduced by one. (City of Calgary)

CBC: Is boring a tunnel out of the question?

Thompson: They may come back and have small segments where they would want to bore a tunnel. That would be a question for those contractors once we get into that piece. But the way we've designed it now is: they have opportunity to use different means and methods.

CBC: But I presume you wouldn't be cutting 4th, 5th, 6th, 7th, 8th and 9th Avenues all at once (to build the tunnel)?

Thompson: No, we would have clauses in our contracts about road impacts, durations for impacts and obviously ensuring that we're not closing all our roads at one time.

CBC: How do you put a tunnel under the existing LRT line on 7th Avenue without shutting it down for an extended period?

Thompson: Again, there's different means that they have. So they can use a tunnelling method. If they were using a cut and cover, they could switch to a different type of tunneling method like a sequential excavation method. They could bring in a tunnel boring machine if they wanted to for a smaller segment. It really becomes the question to the contractors who would be winning that project on the best means and methods.

CBC: Why extend the tunnel all the way through the Beltline as far as the Victoria Park bus barn?

Thompson: Yeah. So we're trading off our costs through the area and we've used lower cost construction for the plan that we have now versus the plan we had before. And so you remember the plan we had before — we had an "S" curve which ran across from the 10th of alignment over to 12th Avenue. We actually had to buy a number of existing buildings, take those buildings down and use a different type of construction. What we're doing now is actually lower costs compared to that original option.

CBC: So even though the tunnel is longer there, you're able to save money?

Thompson: That's right. And we're still looking at details in those areas. And so, if we can, if there is something we can find that we go back to the previous alignment, and we can save more money, we're always looking for those cost savings in the area. But what we've heard from all of our stakeholders is really ensuring that we're not impeding traffic in the downtown, that we're providing good mobility options in the area. There's a lot of buildout that's going to come in that area so we want to make sure that we're providing a lot of options, mobility options into the area.