Mayor Kirk Caldwell is begging the Legislature for another excise tax bailout for Oahu’s runaway rail project, claiming, “This project is just too important to let it die (or) let it stop at Middle Street.”

Lawmakers should make him prove it before dumping more money into this endless fiasco.

The last cost-benefit analysis on rail was back when the projected cost was $4.5 billion — $3 billion from a local excise tax surcharge and $1.5 billion from federal funds.

Expected benefits in terms of ridership and traffic relief are unchanged; if anything, critics contend those numbers are as over-optimistic as original financial projections.

Costs have changed massively. The $4.5 billion is now $10 billion and climbing. The local share is up from $3 billion to $8.5 billion.

Caldwell won’t say what he thinks rail will ultimately cost. Rather than nail it down, he wants legislators to write a blank check for future overruns by permanently extending the half-cent excise tax that was supposed to last only 15 years.

The near-tripling of local costs with no increase in benefits cries for a fresh cost-benefit study to assess if it’s still worth pushing rail to Ala Moana, or if it makes sense to trim back and pursue cheaper traffic-relief measures.

Caldwell is pressuring the Legislature with a federal threat to withhold its funding unless local money covers the deficit by April.

But with the new regime in Washington, who knows if our funding will be released no matter what we do? And it makes no sense to let the feds, whose share is now 15 percent and shrinking, call 100 percent of the shots.

Out-of-control rail is devouring funds sorely needed for other state and city priorities.

In addition to the forever excise tax surcharge, the city is plotting a $65 million increase in gas taxes and parking fees, which usually fund road maintenance, to pay for rail. Higher property taxes and garbage fees for rail are also on the table.

The Legislature, which already scoops 10 percent of the rail excise tax, is threatening to take even more, meaning strapped Oahu taxpayers must not only pay for rail overruns, but also pay more for state services than neighbor islanders.

Our tax base is only so deep, and if every available penny is sucked out for rail, where do we get money for roads, schools, hospitals, decaying infrastructure, affordable housing, and pension debt?

The Legislature should pause further rail bailouts until the city produces a credible bottom-line cost for finishing to Ala Moana and an honest new cost-benefit analysis to prove it’s worth it.

Meantime, the city can use existing funds to get rail to Middle Street, feed it into the bus system and provide commuters a measure of relief.