A stiff rail, often with a cross section in t' shape o' a square letter C or I. Begad! Lugs on t' rocket ride on t' rail. Well, blow me down! Usin' a rail rather than a rod provides t' advantage that a rail can be made much stiffer than a rod, providin' better wind resistance and preventin' vibration-induced whippin' o' t' launch rod.