A cut-and-cover tunnel would connect the light rail line between Commonwealth Avenue and State Circle in the latest plan to overcome the challenges of the preferred route.
Planners are confident they can overcome the challenges of the State Circle route to run light rail past Parliament House.
The ACT is yet to settle on which route will be built between Commonwealth Park and Woden, with a shorter route around State Circle being considered alongside a longer diversion through Barton.
But the government believes it can manage the orderly transition of the line onto State Circle, which runs around Parliament House, in a way that will find favour with the National Capital Authority and Federal Parliament.
Major Projects Canberra has released precinct maps for stage 2B of light rail, offering greater detail on how the network would operate.
A cut-and-cover tunnel would, under the plan, link light rail from the Commonwealth Avenue median under the road between West Block and the Canadian High Commission, and onto State Circle.
Cut-and-cover tunnels are built by digging a trench and then replacing restoring the surface above the newly formed tunnel. The technique is disruptive but generally cheaper than boring a tunnel.
Marcus Sainsbury, from Major Projects Canberra, said the tunnel was a solution to address some of the challenges of the alignment that "we hope we will sit quite lightly in the landscape".
"[It] will be quite simple from a finished product point of view. It will be quite complex to construct, and to construct in a way that doesn't unnecessarily impact the the surrounding traffic," Mr Sainsbury said.
The project is due to be built between 2028 and 2033, the ACT government revealed in February. The government has not said how much the project would cost. It will also include a new bridge over Lake Burley Griffin between the existing Commonwealth Avenue bridge roadways.
Mr Sainsbury said the project was more complicated than the first stage, which opened in 2019 between Gungahlin and the city centre.
"Stage 1 was, you know, comparatively a bit of a walk in the park compared to this project in terms of the generous median that we had to operate in there and none of these complex transitions," he said.
Major Projects Canberra is now focused on developing the draft environmental impact statement, which it plans to finish by the end of the year so it can be released for public comment in 2025.
The government agency expects that process to be completed in the middle of next year, at which point a final decision will be taken on the planned route for the extension.
"I think it's fair to say the preferred alignment is the government's preferred alignment because it's, it's more straightforward," Mr Sainsbury said.
The National Capital Authority has previously warned of the technical challenges associated with the preferred State Circle alignment.
The challenges include managing six lanes of traffic, including two that run up to Parliament House, and three foreign embassies. The route would also need to consider the impact on West Block, a building on the Commonwealth heritage register.
Engineers are still considering whether light rail would run down the middle of State Circle or on the inner edge, closest to Parliament House. No decision has been made on which would be preferred for the route.
Curved 45-metre long platforms would be built on State Circle, which would also require consideration of how to ensure the light rail vehicles meet disability access requirements and minimised the gap between the platform and the carriage doors.
The preferred and Barton routes both include a link in the reserve around the Parliament House precinct to connect with Adelaide Avenue.
The alternative Barton route, along National Circuit, would be expected to present greater heritage challenges, including impacts on the Kurrajong Hotel.
National Circuit is about the same width as London Circuit, but mature trees would need to be removed to make way for the light rail alignment.
Major Projects Canberra has also unveiled a masterplan for Commonwealth Avenue, which includes replacing mature trees. Pin oaks would be planted in the median between the two light rail tracks if the State Circle route is chosen.
Additional street trees will also be planted on the southern part of London Circuit and along the northern end of Commonwealth Avenue.
