The state House and Senate have diverged sharply in their approaches to the budget crisis vexing Oahu’s burgeoning rail-transit system.

The Senate plan is clearly preferable because its limited extension of the tax surcharge Oahu consumers are paying to build the rail would allow construction to continue uninterrupted, without committing taxpayers to a permanent tax hike.

Senate Bill 19, Senate Draft 2, trimmed the extension period proposed in the original bill from 25 years to five, an important change that rightly keeps the focus on completing the original phase of the elevated rail system, which will run 20 miles and include 21 stations from East Kapolei to Ala Moana Center.

By contrast, House Bill 134 would halve the tax surcharge funding the project but presumably extend the life of that tax; it’s unclear because key elements of the bill were left blank to keep it moving and open for discussion. The Senate version’s predictable, limited scope offers the better vehicle.

Since 2007, a half-percent surcharge has been added to the state’s 4 percent general excise tax — bringing the GET tax to 4.5 percent on Oahu — to fund a massive public-works project originally forecast at about $5.2 billion. The surcharge was supposed to expire in 2022. But as construction costs rapidly rise and revenue sources decline, the project now faces a projected deficit of $910 million. The Senate bill would guarantee the GET as a revenue stream until 2027.

Honolulu Mayor Kirk Caldwell and Dan Grabaus-kas, executive director of the Honolulu Authority for Rapid Transportation, the agency overseeing the project, have made a persuasive case that lawmakers should act this session to extend the surcharge to ensure that rail construction continues unabated.

Under the current budget projections, HART would lack sufficient funds to certify contracts that are scheduled to be awarded early next year, causing what are sure to be expensive delays in Honolulu’s overheated construction market.

State law requires that funding guarantees be in place for binding contracts to be awarded. The Legislature can foresee this problem and should act now to prevent it.

That said, Caldwell has overreached in pushing now for a 25-year extension of the tax surcharge that he says would allow the buildout of the original project farther east to the University of Hawaii-Manoa campus and farther west to downtown Kapolei. That is a separate conversation that demands fuller community input, as does the question of how to subsidize operating costs for the rail-transit system once it is up and running.

Ensuring funding for the current 20-mile phase is the urgent question to deal with this session.

Legislators also should act swiftly to lower the untenable "skim" the state government extracts in exchange for collecting the surcharge for the county, and mandate a more thorough accounting from the Department of Taxation about tax revenue generated for the project.

The mayor is to be lauded, though, for stepping up with a full-throated defense of the rail transit project and its importance to improving the quality of life for Oahu residents and visitors who spend far too much of their precious time sitting in traffic.

Rail critics are quick to point out that the system is not predicted to lessen traffic, but there’s no doubt that gridlock will only worsen without it, especially in Leeward Oahu where so much of Oahu’s population growth is oriented.

Honolulu’s City Council must join in the mayor’s vigorous endorsement, including voicing unmistakable support for extending the surcharge for five years, as the Senate bill proposes.

It’s telling that a recent Hawaii Poll conducted for the Honolulu Star-Advertiser and Hawaii News Now found rail and traffic to be the primary concerns for Oahu residents, topping homeless- ness, the economy and education.

Traffic was the No. 1 issue for a full quarter of the poll respondents under age 35, just the demographic most likely to be stuck in it, as they commute to and from work.

The coming elevated rail system and a robust bus system connecting to the train stations are the key elements of an efficient, sustainable mass transit system that will bring relief to those willing to adjust their commuting habits and use it.

HART must keep those future customers in mind as it adjusts station requirements to save money, avoiding changes — such as the substitution of escalators with stairs in some stations — that are likely to depress ridership.

The future of Oahu’s rail-transit project is at a pivotal point. Although the GET admittedly is a regressive tax, the surcharge taps into spending by residents and visitors alike, so remains a better funding source than real property taxes.

A five-year extension — no more — that ensures the project’s completion is the best solution.

There’s no use waiting on an island that already has plenty of skin in the game.