We don't know the ending of this story, but its start is tantalizing.

A man about 40 years old, wearing a windbreaker and carrying a small document bag, got aboard a train at Union Station one recent late afternoon.

The train, a Baltimore & Ohio Railroad commuter express destined for Harpers Ferry and Martinsburg, W.Va., had just made a brief stop at Silver Spring and the conductor, collecting tickets, made the routine announcement that the next stop was Brunswick, Md.

Brunswick? The passenger looked around at the unfamiliar scenery and was assured by the conductor that, indeed, the next stop was Brunswick, not Baltimore.

"But I want to go to Philadelphia," the passenger protested to, obviously, no avail. He had intended to get aboard Amtrak, a few tracks away back at Union Station, but somehow got mixed up.

Brunswick, where he got off, is a nice town with nice people -- but with absolutely positively no way to get back to Washington, let alone Philadelphia, until the next morning, unless one hires a taxicab.

We hope he enjoyed his stay. Friends of St. E's

Friends of St. Elizabeths, a group that supports the nation's -- and the capital's -- pioneering mental hospital, is looking not only at the primary issue of patient care but also at the preservation of St. E's historic and architecturally impressive west campus alongside Martin Luther King Jr. Avenue SE.

The federal government is planning to turn the hospital over to the city by 1988, leaving its physical future in doubt.

Barbara Bick, president of the Friends of St. E's, wrote in the group's current newsletter that the campus could become "a special community within the community: nonhospital, but therapeutic . . . [with] many buildings . . . converted into housing for the mentally ill . . . independent apartments, group homes and unit clusters with common dining and social halls.

"The entire Anacostia community can benefit from our proposal. Center Building can be a superb replacement for the Smithsonian's small Anacostia Museum [now in an old theater in central Anacostia]. The large auditorium and beautiful old theater can be used by local drama groups, schools and clubs.

"Other buildings can be converted into craft workshops, art studios, commercial galleries and shops along the model of Alexandria's Torpedo Factory.

"The real challenge to our vision is the national fiscal crisis . . . . "

Interested? The Friends of St. E's plan a meeting to discuss prospects at 6:30 p.m. April 8 at the King Library, Ninth and G streets NW. A Menace on Skates

Okay, bicyclists, who complain that Metro Scene complains too much about lawbreaking bicyclists who run red lights, violate one-way street rules and illegally ride on downtown sidewalks. Although you're not excused, we've found another villain.

The other evening, at the height of the rush hour, a teen-ager on a skateboard whooshed through a red light at 15th Street NW going the wrong way in a curb lane on one-way-eastbound I Street NW. The skater was oblivious to one oncoming motorist who swerved in apparent panic and almost caused an accident in order to miss him.