The train's a'comin' on Aug. 4 and Aug. 5. Featuring such modern conveniences as air-conditioned coaches, and open-window sightseeing cars, two steam locomotive excursion trains will be chugging their way from Richmond to Keysville, Va. this weekend. Proceeds from advance ticket sales for the trips will go to the National Railway Historical Society, a non-profit organization. Tickets are available from Old Dominion Chapter, N.R.H.S. Box 8583, Richmond, Va. 23226.