AFTER 15 TEDIOUS YEARS of political back-and-forth over how to move people by transit between two glaringly unconnected dots on Metro's Red Line, Montgomery County has to make a pivotal call in a hurry: Either agree on a route and move it along in the first months of this year or forget the whole project for at least five to seven years. Purple Line passions remain high, but without unanimity among the players -- County Executive Douglas M. Duncan, the County Council, the Planning Board and incoming Gov. Robert L. Ehrlich Jr. -- there is no hope of securing federal funding for a cross-county rail project. Will Montgomery leaders talk it to death or sing out with one voice, the only way to get any federal attention?

Mr. Duncan has long favored an outer Purple Line route, a heavy-rail line outside the Beltway that would bypass Bethesda and Silver Spring to serve communities such as White Oak, Wheaton and Grosvenor. This makes long-term sense, as it would put solid transit service in areas where housing and commercial development could be integrated with public transportation. But an inner, light-rail route enjoys strong support from civic and business groups, which cite the transit needs of existing populations. This makes short-term sense, and in the best of financial worlds the inner line would be built also.

To make something happen, Mr. Duncan is floating plans for a compromise route devised by Metro officials at his request. It would use Metro trains running alongside the Beltway to carry riders between the Medical Center and Silver Spring stations. Every official should take a good look. The first look could be by the Planning Board, to review costs, ridership numbers and possible environmental problems. In its favor, the new route would be heavy rail, which could be neatly integrated with the current Metro system. Depending on the number of stations, transit time between Bethesda and Silver Spring could be far less than that of light rail. Another not-so-small matter: Mr. Ehrlich, whose support for any project is essential, has strongly opposed the inside-the-Beltway trolley idea.

It boils down imperfectly to whether Montgomery County wants the train or the issue. Congress is to reauthorize transportation funding this year, and states are readying their proposals now. If Maryland doesn't submit a Purple Line proposal this year, there won't be another opportunity in the near future. Already, Baltimore and Prince George's County have wish lists set. Montgomery must get its act together before the big federal train leaves the station.