Responding to questions about a possible federal government investigation of the proposed tram line between Civic and Woden on Tuesday, Mr Barr said if necessary he would seek a mandate for the proposal at the next territory election in 2020.

This would significantly delay the work beyond the proposed timelines.

The last ACT election, held on October 15, 2016, was described by many at the time as a referendum on stage one of the project.

It is not known what impact, if any, a "mandate" from ACT voters for the Civic to Woden leg would have on federal deliberations on the second stage.

Federal parliamentarians may exert considerable influence over what form stage two will ultimately take as part of the route traverses Commonwealth land controlled by the National Capital Authority.

All building work in the Parliamentary Zone must be approved by both the House of Representatives and the Senate.

While the ACT government has been able to rewrite the rule book to accommodate its broader aims for stage one, that is not the case across the lake.

Mr Barr's apparently off-the-cuff remark came after the former ACT Liberals leader, Senator Zed Seselja, told The Canberra Times he would be pushing to have stage two referred to a parliamentary committee when it went to the Senate.

While, as a long-term opponent who described light rail as a "dud" and said it "did not stack up" in 2015, Senator Seselja has his own agenda, there is much to be said for exposing stage two to an additional level of scrutiny.

Stage one, while originally envisioned as a transport project, quickly morphed into a development and urban renewal initiative to breathe new life into the Gungahlin to Civic corridor.

It has kick-started a long overdue rejuvenation of Northbourne Avenue, which may finally emerge as the iconic gateway to the national capital many Canberrans have always wanted.

Other likely benefits include public housing initiatives that will see tenants rehomed from outdated, Soviet-style, brutalist apartment blocks into modern homes in suburban neighbourhoods.

While there are still questions to be answered about the longer-term cost effectiveness of the Gungahlin route, it also has gained traction from a noticeable shift in our demographic centre of gravity to the north.

This reflects, among other things, land availability, access to the airport and the road links to Sydney and Melbourne.

None of these factors are in play on the southern leg. There are no opportunities to open up land along the route to new development and it is hard to see how an expensive tram service would be able to match a well-planned express bus service on either a travelling time or cost basis.

The real question is whether stage two should be allowed to proceed at all, regardless of the outcome of a future ACT election, unless a compelling economic case can be made for it.