THERE ARE STEAM engines and there are steam engines and then there's Norfolk & Western No. 1218, which is so big you won't believe it, and so long they had to hinge her in the middle to get her around tight curves.

You can see for yourself the last two weekends of July, when the 424-ton, 122-foot locomotive and tender will haul excursion trains from Alexandria and Manassas to Charlottesville and Front Royal.

Built in two weeks during World War II, the Class A 2-6-6-4 locomotives could haul a whole tank division; in happier times they brought the circus to town. No. 1218, sole survivor of the 43 in her class, is bright as new money after a total restoration in the Norfolk Southern's Roanoke steam shops.

Manned by volunteers from three Washington-area nonprofit railfan groups, No. 1218 will run to Charlottesville on July 18 and 26 and to Front Royal on July 19 and 25. There will be open cars for cinder-seekers and closed cars for the clean-minded, with fares of $42 adults, $38 for children 5 to 11 (lap babies ride free). For $80 you get a first-class coach with snacks and steward service.

Tickets are available from Arlington Hobby Crafters, 6176 Arlington Boulevard (U.S. 50, in the Willston Shopping Center). For information, call 656-0466 or 703/765-1628. Or send a self-addressed stamped envelope to Steam Train, Box 4101, Falls Church VA 22044-0101.