NO MEMORY can be complete unless it echoes with the rattle and roar, and the smoke and smell, and the hissing and screeching, and the puffing and pounding, of a steam engine.

They don't make steam engines anymore, of course, but it's still possible to make the memories, because the Norfolk Southern Railroad will be running steam excursion trains from Alexandria's Union Station over the next few weekends.

Starting May 18, there will be a daytrip to Front Royal and three more to Charlottesville, plus a two- day tour that takes railfans through the lush Shenandoah Valley to Roanoke behind one grand old engine and brings them back via the rolling Piedmont behind another.

The slowest and smokiest but probably most scenic trip will be the one to Front Royal, which follows a climbing, curving branch line through the Blue Ridge behind Engine 4501, a 2-8-2 Mikado type of 1911 vintage. As on all the steam excursions, there will be special run- bys so passengers can make photographs, films and tape recordings. A 21/2-hour midday layover in beautiful downtown Front Royal allows plenty of time for lunch and sightseeing.

The Charlottesville trips are mainline rip-roarers, two of them behind No. 611, the sole survivor of the last American steam class. Built in Roanoke in 1950, she's fully capable of hauling a heavy "consist" (string of cars) at more than 100 mph, although she doesn't run anything like that fast on these trips because modern roadbeds aren't built to handle 400 tons of locomotive and tender running at top speed. There's a three-hour layover in Charlottesville, with free shuttle buses available for sightseeing.

The two-day excursion will be behind No. 4501 through Front Royal, Shenandoah and Waynesboro to Roanoke, where through passenges will spend the night in hotels. Next day No. 611 hauls the cars home via Lynchburg, Monroe, Charlottesville and Manassas.

Old hands caution that passengers will not want to wear their best clothes, nor their lightest-colored ones. They will want to bring along caps and goggles.

All of the trips are sponsored by the local (and decidedly nonprofit) chapters of the Railroad Enthusiasts and the National Railway Historical Society, and are possible only because of the minimum rates charged by the Norfolk Southern, several of whose top executives also are steam nuts.

All Aboooard!

Tickets for all trips are available from Steam Trains, P.O. Box 456, Laurel, MD 20707. Except for the two-day trip, tickets may also be purchased at Arlington Hobby Crafters, 625 N. Glebe Rd., Arlington (closed Wednesdays). For information, call: 765-1628, 370-3167, 292- 6481, 656-0466 or 972-5612. The trips:

MAY 18 -- Skyline Limited (Engine No. 4501) from Alexandria to Front Royal (passeners may also board at Manassas). Fare $34 adults, $31 children over 4 (lap babies ride free).

MAY 19 -- Piedmont Limited (Engine No. 4501) from Alexandria (or Manassas) to Charlottesville. Fares same as above, or $80 for first class.

MAY 25 & 26 -- Grand Circle Tour behind No. 4501 from Alexandria to Roanoke on Saturday and home to Alexandria Sunday behind No. 611. Fares range from $95 coach to $195 first class; tickets are sold for all or any portion of the trip.

JUNE 1 & 2 -- Piedmont Limited. Same as May 19 daytrip, but behind No. 611.