While predicting the outcomes of elections is always a risky business, polling would suggest we are headed for a change of government at the up-coming federal election.

While Shorten's commitment makes sense politically and will likely further strengthen the ALP ACT Senate vote. But there are still major questions to answer over the viability of the project before it should be given a green light.

With the route yet to be finalised, it's difficult to know with any certainty what stage 2 might cost ACT taxpayers, although Mr Barr has said previous estimates of up to $1.6 billion are no longer accurate, given changes to the likely route.

To be fair, it isn't all just about the politics. The announcement is a positive one for Canberra given it represents a willingness by a possible future government to make a significant investment in our infrastructure.

This has to be welcome given the Coalition has restricted its city building activities since 2013 to a controversial $500 million reboot of the Australian War Memorial while actively seeking to use Canberra jobs to "pork barrel" electorates in the bush.

The investment also makes a certain sense given Canberra is growing rapidly and traffic congestion is getting worse year by year. While the Barr government has left itself open to criticism over the way in which this project was conceived and has been rolled out, it does deserve credit for planning ahead.

The relative speed with which light rail has progressed is also in stark contrast to the on-again, off-again nature of some major transport projects in Sydney and Melbourne and the moves to establish a second Sydney airport at Badgery's Creek.

That said, the $200 million is, while welcome, a fraction of the possible final stage two project cost. Even after it is factored in and the uncertainty of the final cost, Canberra ratepayers will still be facing a significant cost.

This is a massive spend for a small jurisdiction given it would be on top of the $1 billion already spent in the north. It needs to be weighed up very carefully before any final decision to proceed is taken.

That is why it is so important the Barr government responds to its critics by commissioning an open and transparent cost benefit analysis that would determine whether or not there is a business case.

Given stage one, which serves more suburban areas and was simpler to build, is considered financially marginal at best, it is hard to see how the Woden line, even using the simpler State Circle route and with the promised $200 million, can be justified.

There will also inevitably be further complications. The Federal Government's parliamentary inquiry is still considering responses to its report, the National Capital Authority has issues and there are concerns about how to get the tram across the lake without compromising heritage and aesthetic considerations.

Regardless of whether or not federal funds are available, a clear financial case still needs to be made before committing ACT funds to this major project.