This week saw the official launch of one of the most furious political battles Honolulu will have seen in decades.

Mayor Mufi Hannemann threw down the gauntlet with his announcement that he is officially, firmly and thoroughly committed to building a rail transit system for O'ahu.

And just to make matters uncomfortable, the mayor acknowledged that the full system (not counting rolling stock, some land acquisition, overruns and whatnot) would cost around $4.6 billion. Let's not kid around: Round that number to $5 billion just to begin the conversation.

That would make the rail project Hawai'i's biggest civil construction project ever. For comparison purposes, consider the massive H-3 Freeway, which took 37 years from conception to completion at a price of $1.3 billion.

That's chump change compared with what this rail project will cost.

Critics say Honolulu does not need a rail system; that there are cheaper and more efficient ways of dealing with the growing problem of congestion and the challenge of moving folks from work to home. That may be so, but the political tide is clearly running in the direction of a rail system.

If the decision sticks — and make no assumption that it will — we are in for years of environmental, political and budgetary battling. It's easy enough to define a route. It is another thing entirely to actually build this massive thing through neighborhoods, creating opportunities for some and headaches for others.

The elephant in the living room here is how we are ever going to pay for this project. Make no mistake: The 12 1/2 percent excise tax increase approved by the Legislature and the City Council, combined with whatever federal funds we get, will not pay for the full system Honolulu must eventually have.

Hannemann acknowledged as much, saying the general excise tax hike "is not going to get us there."

The idea that the Legislature and the council would go for yet another tax hike is a virtual nonstarter. Lawmakers amazed themselves when they approved the county-option tax increase, and Gov. Linda Lingle surprised many when she let it become law against her better political instincts.

Don't expect that to happen again.

So, how do we make this work?

Hannemann has been talking more frequently about public-private partnerships, and he may have more in mind than has been publicly discussed to this point. If the rail system originates or ends at major regional shopping centers, for instance, shouldn't the private owners of these facilities cough up some serious dough to make the thing happen?

That's been the approach of rail system managers in other jurisdictions, such as Hong Kong, for instance.

And each rail station has the potential for stimulating major development activity around its hub. Shouldn't the public, the taxpayers, capture some of this value as a way of offsetting the cost of this massive project?

And while this is a city project (with some federal help), there is a strong belief that the state may be asked to come up with some of the capital costs, particularly when it is of direct benefit to state institutions. For instance, if a spur is built to the University of Hawai'i-Manoa, or if the line connects to a new state campus in Kapolei, wouldn't it be logical to ask the state to help front the costs?

Bottom line: This is the beginning of a huge political conversation that will involve leaders at all levels of government for decades.

Reach Jerry Burris at jburris@honoluluadvertiser.com.