The tunnel is now used exclusively by runners, cyclists and walkers on the trail between Silver Spring and the Bethesda Row shopping and entertainment district.
State transit planners have said for years that squeezing two train tracks, passenger platforms and a 12-foot-wide trail through the tunnel would be difficult in such a tight space.
Current plans call for the trail to run above the trains in the tunnel. Once the trail exits the tunnel, a series of ramps would bring cyclists and runners back down to Woodmont Avenue.
But MTA officials say more detailed engineering has revealed potential problems with the tunnel design, including the need to strengthen or rebuild at least 35 columns that support the Apex office building on the west side of Wisconsin. Excavating to lower the train tracks to provide enough head room for trail users also would be costly, transit planners said. The tunnel also runs beneath the Air Rights Center’s office building on the east side of Wisconsin.
“It’s challenging because you’re looking at a very confined space and trying to put two facilities through there at the same time,” said Greg Benz, a Parsons Brinckerhoff consultant on the Purple Line project.
The MTA has asked Montgomery County to decide whether it still wants to build the trail in the tunnel. County officials say they would be responsible for paying for the trail construction, but they would seek some money from the state.
Montgomery planners are reviewing the MTA’s findings for the planning board, which will make a recommendation to the County Council.
Officials from the MTA and Montgomery have long promised that the popular trail would be rebuilt if light-rail trains ever run east-west on a 16-mile Purple Line between Bethesda and New Carrollton.
The federal government recently approved the light-rail project for more detailed engineering, but no plan exists to pay for its estimated $1.93 billion construction costs. The trail costs would be in addition to the light-rail line construction.
The state’s latest cost estimate puts trail construction at $93.9 million. But if costs for safety lighting, landscaping and benches are included, the total increases to $103.3 million. About 43 percent of the total trail costs stems from the 1,000 feet inside the tunnel, planners said.
The other option would be to reroute the trail onto local streets to avoid the tunnel, requiring trail users to cross Wisconsin.
Purple Line planners have said the trains must enter the tunnel to provide a direct connection to new elevators that will carry passengers between the street-level Purple Line and the underground Bethesda Metrorail station.
Montgomery council member Roger Berliner (D-Potomac-Bethesda), whose district includes the trail, said the county remains committed to rebuilding it, but cost will be a factor.
“We’ll have some interesting negotiations ahead with the state as to what its responsibilities are and what our responsibilities are,” Berliner said.
Trail advocates say keeping the trail in the tunnel is vital to allowing trail users, particularly young children, to cross safely beneath busy Wisconsin.