Wednesday’s formal signing ceremony was a momentous occasion for Hono-lulu, with the long-awaited federal full-funding grant — $1.55 billion toward the $5.26 billion cost of the elevated rail project — winning final approval.

It would have been a momentous day, too, for U.S. Sen. Daniel Inouye, whose death two days earlier added poignancy to the event.

Inouye, the Senate appropriations chairman, was the project’s most influential champion in Washington, D.C., at a fiscally strained time when such capital investments really need a champion.

The late senator, who was to lie in state today in the U.S. Capitol, was represented at the ceremony on The Hill by his widow and a lei-draped chair.

This is far from the last hurdle the Honolulu Authority for Rapid Transportation will have to surmount.

But for the supporters of the project, which has the potential to equip Honolulu with the robust transportation network it badly needs, this was a day for celebration.

About those hurdles: The one in clearest view is set in federal court. Last week Judge A. Wallace Tashima of the U.S. 9th Circuit Court of Appeals heard oral arguments in a case that opponents hope will effectively halt the project.

The plaintiffs group — which includes former Gov. Ben Cayetano, longtime rail opponent Cliff Slater and Hawaii’s Thousand Friends — had sought an injunction earlier, arguing that the project violated federal laws aimed at protecting parks and historic sites.

But Tashima’s initial ruling Nov. 1 was more measured, ordering the city to supplement its study of possible historic-site protections and setting last week’s hearing. At issue is whether that order means delaying work on the whole project or, as the city prefers, only the downtown phase where the most sensitive historic sites, principally traditional Hawaiian burials, are located.

It seems almost certain that getting the federal grant approved in advance of Tashima’s decision was a strategic move, a signal to the judge of congressional and top-level administrative support for the project. And, if that’s so, getting the project past what may be the point of no return would be Inouye’s final push for rail, which he had avidly backed.

In August the senator, then 87, toured the West Oahu construction site, where columns and foundations for the first elevated section had been under construction since April, and acknowledged that he had been working on behalf of rail for most of his political life. The fact that it has taken this many decades — city officials had backed away from the project twice previously — underscores the misgivings that many Honolulu residents still have about it. For that reason, many have doubted that the project, despite the considerable resources behind it, would ever reach this point.

Though now it has arrived at this key juncture, the project still needs buy-in from Oahu residents. The various legal challenges halting work have compounded some of its costs, and the taxpayers should be watching even more carefully to see that public funds stay on target.

It now falls squarely on the rest of the project advocates — in City Hall as well as Capitol Hill — to step up and ensure that rail development hits the many marks that lie ahead.