There’s a reason why a fallback option is called “Plan B.” It’s plainly not as good as Plan A.

In the case of the city’s elevated rail project, Plan B would terminate the guideway in downtown Honolulu but would sacrifice transit stops that almost certainly would diminish its usefulness to a large percentage of potential riders.

Describing just how far short it falls as an alternative is the job of advocates for a truly serviceable rail system, as lawmakers meet to evaluate the way forward on the city’s project, deeply mired in financial trouble.

Of course, this means the Honolulu Authority for Rapid Transportation and city officials will have to make the case for why additional revenues to overcome a funding gap are needed to produce a result worthy of the investment.

The coming legislative session is a make-or-break time for HART, which has been given an extension of April 20 to devise a new financial plan to cope with astronomically higher cost projections for completing the project as envisioned, extending from East Kapolei to Ala Moana Center.

There is $6.8 billion available through the existing mechanism of a temporary half-percent surcharge on the general excise tax Oahu residents pay. Current projections put rail’s cost at around $3 billion beyond that, and covering that bill would require a further extension of the GET surcharge.

Legislators need to face the fact that this is the only acceptable route to a solution. “Building to budget” works, but only if the result is something that serves a purpose.

The problem with the new deadline is that it would require lawmakers — and the governor — to act quickly so that HART can finalize the plan by the federal deadline.

They should approve an extension of the GET surcharge, which is the most efficient means of tapping residents and visitors alike, most of whom will either ride the rail or benefit from the economic infusion, redevelopment and transportation improvement.

The bottom line is: That’s what it will take to enable completion of Plan A.

Let’s consider the alternative.

Plan B is to get commuters to downtown by building at least the rail guideway to Aloha Tower. According to the interim plan prepared for federal authorities in September, this would require “deferral” of the Pearl Highlands Transit Center, along with the Kalihi, Kapalama, Iwilei and Chinatown stations.

Deferral means that it won’t be built until it’s much costlier to do so, which leaves prospects for their completion looking fairly murky. In the meantime, the rail system might serve commuters heading to work downtown, but not if they start or end their trip between Middle Street and Aloha Tower.

Even setting aside the loss of ridership from those who would like a trip to the commercial hub of Ala Moana, that means a lot of residents who could use the system likely will stick to car or bus travel. And that means many more cars in the congested city center.

Further, the loss of the Pearl Highlands stop, with its parking garage and H-2 freeway off-ramp, signifies that the city would be giving up on the notion of Central Oahu residents being served by the rail.

Taxpayers must consider: $6.8 billion is cheaper than the $9 billion-plus projected for the full buildout, but the end product does not fulfill the city’s needs.

Finally, there has been a lot of complaining that the April deadline won’t allow the state its full allotment of time to ruminate over this. The response to legislators should be: Oh, please, get real. Now is the time to do not what is usual, but what is required. To do their job, in other words.