FOR THE time being, Gov. Martin O’Malley (D) can bask in headlines about the light rail Purple Line, which he jump-started Monday by announcing Maryland would add $400 million in state funding and seek a private partner to build and run the project, which would connect Montgomery and Prince George’s counties. He can leave it to another day to worry about financing the rest of the $2.2 billion construction, which still looks iffy. Or, just as likely, he can leave the worrying to his successor.

The Purple Line, which has been on the books for more than 20 years, is an important project that could ease traffic and spur development in Maryland’s inner suburbs. Critically, although it is not part of Metro, it would link Metro’s Green Line, Orange Line and both arms of the Red Line. That would enhance mobility for tens of thousands of commuters.

The infusion of funding is a function of Mr. O’Malley’s success this year in enacting a sharp increase in the gas tax, which is expected to yield more than $4 billion in transportation revenue over six years. Nonetheless, the project is a long way from a done deal.

In the rosiest scenario, construction would begin on the 16-mile Purple Line in 2015, and it would start carrying passengers in 2020. In reality, that timetable would require something approaching a miracle, especially in this sequestration era in Congress.

Under the financing scheme advanced in Annapolis, the feds would contribute $900 million to build the Purple Line, nearly half its total cost. Unfortunately, that’s probably a pipe dream — it’s the same amount Congress approved for Metro’s Silver Line extension in Northern Virginia, whose cost is almost triple the Purple Line’s.

Further complicating the Purple Line’s chances of attracting major federal funding is the fact that Mr. O’Malley has so far declined to assign it higher priority than a transit project in Baltimore known as the Red Line, whose project cost is pegged at $2.6 billion. Federal transportation officials are unlikely to lavish increasingly scarce funds on competing transit projects in a single small state.

If there is hope for the Purple Line, it is Mr. O’Malley’s decision to forge a partnership with a private firm that the state would select to build, operate and maintain it. The experience with such arrangements, in Virginia and elsewhere, is that private partners are effective at getting projects done quickly, and enlisting local developers — for example, near some of the Purple Line’s 21 stations — to help pick up the tab.

A private partner might also make the Purple Line a better bet to attract federal funding, even if not the full $900 million the state would like . State officials say Baltimore’s Red Line is less likely to be financed in full partnership with the private sector.

Virtually all transit systems require government subsidies; the Purple Line would, too, in the form of an annual payment to the private operator. Most appealing to future passengers is Maryland’s apparent determination to use state funding to hold the operator to performance standards — for example, cutting subsidies if it falls short on customer satisfaction, on-time performance or cleanliness.

The project remains shrouded in unknowns, and not just the federal role. Will a suitable private operator be found? Will the state, whose contributions now total $680 million, dig even deeper into its pockets? How much local funding will taxpayers in Montgomery and Prince George’s be asked to kick in?

Through the inevitable setbacks and funding travails, it’s important to remain mindful of the Purple Line’s promise: to knit together a transit system that until now has been inadequate to the needs of a fast-growing region.