The Downeaster, Maine’s Amtrak connection to Boston and the rest of the U.S., is an unequivocal success, with passenger numbers exceeding the projections of even its most ardent boosters. In just six years of service, the train is approaching boarding its 2 millionth passenger, a milestone that was expected to be reached three years from now. Gov. Baldacci and legislators must keep up the head of steam the railroad has built in recent years, even if it means tapping scarce state funds.

The train service has received annual funding from the U.S. Department of Transportation’s Congestion Mitigation and Air Quality Improvement program. That funding, which the Downeaster won when it began, and then was renewed thanks to the efforts of Sens. Susan Collins and Olympia Snowe, is set to expire in 2009. To make up for the missing money, the state needs to come up with $7 million to $8 million annually. A task force had recommended raising the tax on car rentals, which would raise about $5 million annually, but that recommendation has been set aside. It ought to be revisited.

Critics are quick to object to federal or state money propping up a service which, they would argue, should rise or fall on its support from the market. Fares, not tax dollars, should fund the service, they argue.

But proponents correctly note that all transportation infrastructure – roads, bridges, airports, sea ports, ferries and ferry terminals – is sustained, at least in part, by public funds. And the benefits of having the train link from Portland to Boston are many. Traveling by train – or by the Trailways bus, which is a private sector companion to the Downeaster – is cheaper than traveling by private vehicle, and becomes more so every day as gas prices rise. By parking cars and riding in a train, pollution is reduced. And the regular service, which is fast and safe, is something commuters can count on, effectively making parts of York and Cumberland counties bedroom communities for those who work in Greater Boston. Large-scale development projects under way in Saco, Freeport and Brunswick hinge on the service continuing and expanding northward.

If the Downeaster continues to thrive, further links to Brunswick, Rockland, downtown Portland, Lewiston-Auburn, Augusta and then Bangor and Montreal could soon follow. Though Mainers often feel they live on the nation’s cul de sac, the state should instead see itself as part of the populous Northeast U.S., a status other rural but more isolated states would envy for its economic benefits.

The sad reality is that federal transportation funds are trickling, rather that streaming back to Maine, a state of affairs that is affecting road and bridge work, as well as rail. That status is not likely to change, given the federal government’s deficits. Because users now count on the service in its current form, cutting back on trips is out of the question. Supporting existing service, and even expanding it, are worthy of state support, even if it means a tax hike on car rentals.