Approximately 27 miles of the 75-mile double-track San Francisco Bay Area Rapid Transit System will be carried on standard aerial structure. The typical aerial structure consists of two parallel girders of uniform cross-section simply supported on single-column piers. The principal elements of the standardized aerial structure are girders of prestressed concrete and composite steel-concrete. This paper is concerned primarily with the design and use of prestressed concrete for the girders. Each trapezoidal box girder is a structural entity supporting one track. In plan, the webs of each girder are straight between piers. The top slab, upon which the rails are mounted, follows the curvature of the horizontal alignment. In elevation, the entire girder parallels the rail profile. Given the stations of the piers and the alignment geometry, computer programs develop pier construction controls and all variable girder dimensions. Designs for the prestressed concrete girders permit several alternative methods of prestressing. The girders may be post-tensioned with cables or rods, or pretensioned. Each permissible prestressing system is completely detailed on standard plans which are used for each construction contract. Standard designs have been developed for the use of prestressed concrete girders for pier spacings between 40 and 100 feet. The concrete has a 28-day design strength of 6,000 psi and must meet specifications which limit shrinkage and creep deformations.