Montgomery County officials opened a tunnel in Bethesda yesterday, giving pedestrians and bicyclists who use the Georgetown Branch and Capital Crescent trails a safer way to cross Wisconsin Avenue.

The tunnel also will connect the two trails, nearly completing a 12-mile, paved thoroughfare that runs from the Potomac River in Georgetown to Silver Spring.

The 800-foot-long tunnel runs below the Apex and Air Rights buildings and curves slightly in connecting the east and west sides of Wisconsin Avenue, enabling trail users to bypass car traffic and stoplights at the intersections created by Wisconsin, Woodmont and Bethesda avenues. The tunnel was once used by trains ferrying coal to heat much of the Washington area.

"It is important for more recreational bikers to have a clear crossing of Wisconsin," said Ray Wisher, a Bethesda bicyclist. "It's dangerous for me to cross Wisconsin."

The tunnel will be open daily from 6 a.m. to 10 p.m. and will be part of the Montgomery County police bicycle patrol, county officials said. After hours, the tunnel will be closed by chain-link fencing topped with razor wire.

People who enjoy the trails said the tunnel would encourage more people to use the pathways.

"This is great; you see people out here you think never leave their homes," Bethesda jogger Marilyn Ochs said.

It took two months to pave the tunnel's pathway, paint over its graffiti-marred walls and install a chain-link fence along the trail to keep people out of nooks and crannies. The county ponied up most of the $455,000 cost, but the Capital Crescent Trail Coalition, a lobbying group working to expand the Washington area's trail system, chipped in $45,000.

The tunnel features a well-marked path illuminated by decorative street lamps. To the east, the Georgetown Branch Interim Trail heads into Elm Street Park. To the west, the Capital Crescent Trail leads to shops and other businesses on Reed Street.

"This has been part of our master plan for years: to open more pedestrian walkways and paths for bikers," said Montgomery County Executive Douglas M. Duncan (D). "We are trying to give people an option to walk or bike and not just use their cars."

The tunnel connection is part of a larger plan to create a "bicycle beltway" around the Washington area, said John Dugger, chairman of the Capital Crescent Trail Coalition.

The beltway trails "are, in a sense, linked together now," Dugger said, "but every little link helps" make the system easier to navigate.

The county purchased the eight-mile swath of the former Georgetown Branch rail line from CSX Transportation in 1988 for transportation purposes, possibly to accommodate a light rail line between the Bethesda and Silver Spring Metro stations. The tunnel's opening for use as a trail connector caused concern among supporters and opponents of such a commuter line. "The light rail {opponents} are worried about a little bait and switch," Ochs said. "Now that {county officials} have the tunnel open, it could be used for the light rail, which not everyone is happy about."

But David Weaver, a county spokesman, said some light rail supporters worry that the pedestrian trail might "preclude light rail" from being built.

In any case, he said, currently there are no plans to study light rail. Georgetown Branch Trail Tunnel The tunnel, which once was for trains, is 800 feet long and runs beneath the Air Rights Building and the Apex Building. It will be open every day from 6 a.m. to 10 p.m. (This graphic was not available) CAPTION: The 800-foot-long tunnel enables trail users to bypass congestion at the intersections created by Wisconsin, Woodmont and Bethesda avenues.