A shrill whistle pierces the early-morning air. Clusters of beaming faces pass by the window like frames of a film. The clicking of the wheels against the track reels off each frame with increasing speed. Suddenly the faces disappear, the trailing edge of the platform flicks away and you're off on a steam- train journey into the past.

Your chance to take the trip begins this weekend, with the first two of seven summer excursions featuring vintage steam-powered locomotives and sponsored by the Washington and Potomac chapters of the National Railway Historical Society (NRHS) and the Chesapeake Division of the Railroad Enthusiasts Inc. There are three destinations to choose from: Front Royal, Charlottesville and Hagerstown. Each route boasts its own distinct attributes: hazy Blue Ridge foothills, lush Virginia farm country or the northern Shenandoah Valley. Your destination may not justify the $30 cost per person, but it's how you get there that's the attraction.

As you leave Alexandria's Union Station behind, you can sense the excitement of fellow passengers. For many it's their first train trip. Others are veteran trainers, members of the NRHS and Railroad Enthusiasts who staff the train as conductors, commissary car attendants and ticket-takers. They also serve as fountains of railroad knowledge -- but you can't sip without getting soaked. Their enthusiasm is infectious as they debate such issues as the merits of diesel power over steam, or the degree to which the federal government's policy toward the trucking industry led to the end of the golden age of the railroads. One old-timer who's spent more than 60 years of his life on the rails speaks reverently about departed lines and is obviously enjoying his busman's holiday.

Suddenly he says, "Feel that? The engine -- she's really working hard now!" You stand motionless for a moment, straining all your senses. Nothing. Nevertheless, you politely nod in agreement, remembering the sound in your car engine that only you can hear.

Young enthusiasts exhibit a more contemporary interest in trains. One is talking about his extensive library of train films and soundtracks that he's made on previous excursions. He's explaining how he once got revenge on his loud-music-loving neighbors by placing his stereo speakers flush against their common apartment wall and running a K-2 locomotive through their living room.

Passengers get a chance to record the trip for posterity. At a predetermined, strategically located stretch of the tracks, you get off the train for what is known as a photo run-by. At a safe distance from the tracks, called the firing-line, you wait with camera poised. The train backs up around a turn, then comes roaring round the bend. The engineer has pulled all the stops. Blowing the whistle, belching cinders and spewing a thick black plume of smoke, the iron monster roars past you like a page out of history.

Afterward, you understand why hundreds of onlookers have been staring and waving at the train at crossroads and nearly every spot with a view of the tracks. Babies are hoisted onto shoulders for a view and cars race the train along back-country roads paralleling the tracks. As you wave back, you find that, although fun, greeting train fans from an open-air coach has its drawbacks. Along with the taste of nostalgia comes a taste of cinders, which bombard your skin, clothes and eyes with relentless gusts. Coupled with the heat, the effect is an uncomfortably gritty feeling, somewhat akin to fighting a forest fire.

For relief, you can head to the well- stocked commissary car where iced tea and a sandwich taste as good as a hotdog does at a ballgame. When you return to claim a seat, you may be inclined to find one in one of the standard air-conditioned or first-class coaches.

From here you can concentrate on a view of American life that can be experienced only through the window of a train: From the industrial heart of cities, tenements and the backyards of suburbia into farmlands, forests and open country, your train whistles behind the stage props of society, and America is caught with her hair down. GETTING INTO TRAINING VIRGINIA STEAM TRAIN EXCURSIONS -- leave Alexandria Station at 8 a.m. for Front Royal this Saturday, Sunday and July 2; for Charlottesville on July 3, 9 and 10; and for Hagerstown on July 16. Prices start at $30 for an adult; $27 for children five to 11. Group rates are available. First-class and reserved seats must be ordered by writing Steam Train, P.O.Box 762, Gaithersburg 20877. Open-seating tickets may be purchased at: ARLINGTON HOBBY CRAFTERS, 625 North Glebe Road, Arlington. 522-6442 (Closed on Wednesday.) CRAFT CORNER, 2918 Dale Boulevard, Ashdale Plaza Shopping Center, Dale City, Woodbridge. 703/690- 2525. HOBBY WORKS, INC., Laurel Center Open Mall, Laurel. 301/953-2566 (Washington), 301/792-2556 (Baltimore).