They called it an incomparable opportunity, a chance for Montgomery County to buy a 6.4-mile abandoned rail spur arching from the District line through Bethesda to Silver Spring. This ribbon of land would connect the Chesapeake & Ohio Canal to Rock Creek Park, and it could pave the way for the area's first light rail transit system.

So, in a special session yesterday, the Montgomery County Council voted to pay $10.5 million to buy the Georgetown spur from CSX Transportation Inc., which last ran a freight train on the track in March 1985.

While agreeing to preserve the corridor for public use, the council has yet to decide exactly what to do with the land: use it exclusively for recreation as a hiking and bicycling trail or to combine a park with a cross-county trolley line linking Bethesda and Silver Spring.

County Executive Sidney Kramer and the county's transportation planners support construction of the transitway, which they estimate would cost $47 million and would attract 17,000 daily riders by the year 2005. They hope to persuade the state, which has authorized construction of a light rail system in Baltimore, to pay 100 percent of the construction costs.

Some local citizen groups, including the Greater Bethesda-Chevy Chase Coalition, oppose the line, saying the county has inflated projected ridership while underestimating the cost, which they put at $100 million.

Council members, who will decide next year whether to build the trolley line, took less than 10 minutes yesterday to decide unanimously to authorize the $10.5 million emergency expenditure from the county's surplus.

"I said we could do this in three minutes . . . We really didn't have a choice," said council member Rose Crenca, who called the land a "priceless heritage."

Council member Bruce Adams said the purchase was "the modern day equivalent" of decisions to buy the land for Rock Creek Park or to preserve the C&O Canal as a national park. "Imagine how less glorious our life would be" without those parks, Adams said.

The purchase, set for completion next week, caps nearly three years of negotiations. At one point, CSX was demanding $60 million for the 60-acre right of way and it looked as if the land might be sold off piecemeal. County officials credit developer and businessman Kingdon Gould Jr. with breaking the impasse by becoming involved in the negotiations with CSX and the Interstate Commerce Commission. Gould purchased from CSX this week an option to buy the 4.6-mile portion of the spur that lies along the Potomac River in the District.

Gould said he hoped to negotiate an agreement with the National Park Service for purchase of the land that would commit it to public use. A congressional appropriation would be required.

Chris Brown, chairman of the Coalition for the Capital Crescent Trail, which has been working to make the corridor a public greenway, called the Montgomery purchase "a dramatic and nationally significant commitment to conservation by a local government." Members of the Greater Bethesda-Chevy Chase coalition that opposes the proposed trolley also praised the council's action.

County Transportation Director Robert S. McGarry, who is spearheading efforts for the trolley, said he would be presenting the council more information on projected costs and ridership. The earliest the trolley could be running is five years, he said.