For photographers who want dramatic slides or prints of steam locomotives, or railfans who just like old trains, there are special Virginia excursions coming up this weekend and next.

Few sights match the dramatic approach and passage of a steam locomotive, drive- rods churning, smoke and steam rolling. With some attention and care, the spectacle can be captured dramatically on film. There will be four excursion trains drawn by a mammoth 2-8-4 "Kanawha" steam engine from Alexandria to Charlottesville and back via Southern Railway's main line, passing through Manassas, Culpeper and Orange Court House.

The photographer has two choices on each day -- ride the train and disembark for the two scheduled photo run-bys, or scout out photo locations by train or car.

Of great help in scouting are the Washington and Charlottesville maps in 1:250,000 scale, available at $3.25 each from the U.S. Geological Survey map stores at 19th and F Streets NW and at 1200 South Eads Street, Arlington (near the Pentagon).

The maps show three locations where the train rounds picturesque curves: between Clifton and Manassas Park; at Orange Court House; and just northwest of Burnleys, in Albemarle County. And there is a grand straightaway run through Brandy Station, south of Warrenton.

The locomotive is always rushing away from the sun, which calls for shading the lens, and there are several other pitfalls.

The first and most common is "buck fever." Almost all first-time photographers of steam trains, carried away by the drama of the moment, shoot too soon. The cure is to concentrate on the image in the viewfinder, after having determined the composition of the photograph in advance.

Failing to comprehend the speed at which the locomotive is moving, with the consequent need for a shutter speed of at least 1/250 second, is the second pitfall. Veterans use 1/500 when they can.

The final pitfall is failing to compensate in exposure for a black locomotive pouring out black smoke against a green forest. If the locomotive and train are the principal subject, open up half a stop; if the train is to be part of the landscape, normal exposure will be adequate. A normal lens (50mm for the 35mm camera), yields excellent photographs when the pictures are composed beforehand. The engineer and members of the sponsoring organizations make every effort to accommodate photographers. In the photo run-by, the train is stopped, all who wish to do so disembark and assemble behind a photo "firing line." The train is backed down the track and then driven past twice, once at a relatively slow speed and then faster. The engineer provides quantities of black smoke and a heavy hand on the whistle for sound movies and recordings; experienced photographers offer advice. The sponsors are the non-profit Washington and Potomac Chapters of the National Railway Historical Society, and the Chesapeake Division of the Railroad Enthusiasts. During the four-hour lunchtime layover in Charlottesville there's time to take in a city arts festival that puts special emphasis on railfan activities. TAKE THE S TRAIN

The speciasl steam excursions will run July 17, 18, 24 and 25. Round-trip coach fare between Alexandria (or Manassas) and Charlottesville is $29 for adults, $26 for children. A $5.50 box lunch may be ordered at ticket purchase. Trains leave Alexandria at 8 a.m. and Manassas at 8:45; leave Charlottesville at 3:30; return to Manassas at 5:45, and Alexandria at 6:30.

Tickets may be purchased at Arlington Hobby Crafters, 625 North Glebe Road (522-6442); PD Craft House, 8701 Colesville Road, Silver Spring (585-4224); and Craft Corner, 2918 Dale Boulevard (Ashdale Plaza), Woodbridge (703/690-2525. PHOTO CONTEST -- George Washington's Birthplace wants some nice photographs of itself. Entries may be in black necessityand white or color (5x7 or 11x16), must be taken in the park, and must be of: buildings and structures; scenery; animals; or living history. Deadline is July 31. Write Photography Contest, Washington's Birthplace, Virginia 22575, or call 804/224-0196.