Montgomery County, Md., is determined to ensure that the new Purple Line light-rail system will join the dozens of active rail lines already sharing rights-of-way with recreational trails across the country.

For this to happen, however, the County must first determine the best way to execute a multi-modal squeeze play.

A transit counterpart to the Inter County Connector, which is finally nearing completion a half-century after its conception, the 16-mile Purple Line, administered by the Maryland Transit Administration (MTA), aims to expedite east-west travel across Washington, D.C.’s northern suburbs by connecting the edge cities of Bethesda and New Carrolton.

Part of the route incorporates a segment known as the Georgetown Branch, a one-time heavy rail line purchased by Montgomery County in 1988 that has since become among the area’s most popular recreational trails.

Though a transit use has always been in the cards for the Georgetown Branch, Montgomery County has always promised to rebuild any sections of the trail displaced by the Purple Line, allowing the corridor to eventually become fully integrated with a regional trail network.

Even with these assurances, the potential marriage of a revived rails and trails has not been without controversy over the past 20 years, with trail users worried about the safety and loss of mature trees. But it took the Federal Transit Administration’s go-ahead for detailed engineering of the Purple Line in early October to confront that lofty vision with some potentially costly technical realities.

The problem is an existing 1,000-ft tunnel that funnels the trail beneath a portion of downtown Bethesda. Also envisioned as part of the Purple Line route, the tunnel would include a platform where riders could take elevators to street level and the nearby Bethesda Metrorail station.

As the tunnel’s current clearance is only 8-9 feet, excavation by another 8-10 feet will be necessary to accommodate the tracks, platform, and the 12-foot wide trail, which according to plans would be routed on an overhead structure.

However, an engineering study has determined that doing so will also require reconstructing 35 existing columns supporting a 123,000-sq ft office building above the tunnel, plus the relocation/replacement of the bracing grade beams supporting an adjacent surface street.

The study estimates that this dual-use strategy would add approximately $40.5 million to the estimated cost of rebuilding the trail for the Purple Line, bringing the total price tag to $93.9 million. (In other words, 43 percent of the cost would be devoted to less than 4 percent of the trail.)

That’s on top of MTA’s projected $1.93 billion Purple Line construction cost, for which there is currently no funding.

To be sure, tunnel modifications may be necessary if the County elects make it transit-only, relocating that section of the trail to the surface . But that creates a new problemhow to keep bikers and walkers (many with small children) safe on their way through downtown Bethesda, particularly where they’d have to cross busy Wisconsin Avenue, the city’s main thoroughfare.

An Wisconsin Avenue overpass might be the answer, but that’d entail not only yet another structure, but approaches that’d have to be integrated into the existing cityscape.

While Montgomery County ponders this issue, MTA is searching for funding sources that will take the Purple Line off the drawing board and into construction.

Though FTA New Starts funding is always a possibility, project proponents are facing up to the reality that private-sector involvement may be necessary. A proposal by a Maryland General Assembly panel to boost the state’s gas tax by 15 cents would not affect either the Purple Line or a proposed expansion to Baltimore’s Red Line system.

UPDATE 11/18/11: Montgomery County Planning Board asks MTA to consider alternatives to Purple Line routing, station location.