A rendering that depicts a Purple Line tunnel that would run beneath Bethesda. (Courtesy of Maryland Transit Administration/COURTESY OF MARYLAND TRANSIT ADMINISTRATION)

Rebuilding the Capital Crescent Trail through a tunnel in downtown Bethesda as part of a Purple Line transitway would require funneling runners and cyclists through an eight-foot-tall clear tube built above light-rail trains, state engineers told local planners Thursday.

Engineers for the Maryland Transit Administration, who toured the tunnel, said they would have to elevate a new 12-foot-wide trail 10 feet above its current location and enclose it in glass or another clear material to let in sunlight while protecting trail users from the light-rail trains’ overhead power lines. The tube would run above the trains — which the state’s Purple Line project manager said is doable, although he hasn’t seen it done anywhere else.

When asked how enjoyable the enclosure would be for users of an outdoor trail, Michael Madden, the state’s Purple Line study manager, said, “That’s a valid point, and what we’re bringing to the county’s attention. There are all kinds of costs and risks associated with getting it through the tunnel.”

How to squeeze the trail, two train tracks and station platforms through a tunnel that now carries only the trail beneath Wisconsin Avenue and two office buildings would be the most difficult part of building a 16-mile Purple Line between Bethesda and New Carrollton, Madden said.

State transit planners conducted a tour of the tunnel near the Bethesda Row shopping and entertainment district for Montgomery County planning board members, who will consider the trail’s design Nov. 17. About 50 people accompanied the members, including state engineers, local transportation planners and activists holding up green “save the trail” signs.

The tour followed a state report released last week that says cost estimates to rebuild the trail between Bethesda and Silver Spring had risen to $103 million, up from a 2010 estimate of $65 million. The county is responsible for the trail costs. The state is seeking federal funding for half of a Purple Line’s estimated $1.93 billion construction costs. The state would pay the other half.

The report says that about 43 percent of the estimated total trail costs stem from trying to keep it in the 1,100-foot tunnel rather than rerouting it along streets through downtown Bethesda. Rebuilding the trail in the tunnel would also increase the risks, planners said, because it would require significant excavation around 35 columns and other critical beams supporting one of the two office buildings above.

“We’ll have to monitor those buildings very carefully [during construction] to make sure they’re not moving,” Madden said. “If they do, we’ll have to evacuate those buildings.”

Ajay Bhatt, president of the nonprofit group Friends of the Capital Crescent Trail, said that local officials need to keep long-held promises that the trail will remain in the tunnel to keep cyclists and runners from having to cross busy Wisconsin Avenue.

“I can’t imagine people would enjoy that trail experience,” Bhatt said of an enclosed trail above trains, and “it seems like it will be a safety hazard” by attracting crime.

Beyond the Bethesda tunnel, most of the trail would run adjacent to, and about three feet above, the light-rail tracks, state engineers said. Trees and bushes would be planted to provide a buffer between trains and trail users.

The tour also highlighted lesser-known choices that Montgomery officials will have to make about how much they want to spend and how much environmental damage they are willing to allow in Rock Creek Park to connect a trail along the Purple Line with the popular Rock Creek Trail.

The north-south Rock Creek Trail runs far below where an east-west trail along a Purple Line would cross Rock Creek on a bridge east of Jones Mill Road. Connecting the two trails via a proposed series of switchback ramps would require cutting an unknown number of trees in Rock Creek Park, state engineers said. It also would be more expensive than building and improving sidewalks along the current trail connections on local streets, planners said.