5XB was originally intended to bring the benefits of crossbar switching to towns and small cities with only a few thousand telephone lines. The typical starting size was 3000 to 5000 lines, but the system had essentially unlimited growth capacity. The earlier 1XB urban crossbar was impractically expensive in small installations, and had difficulties handling large trunk groups. 5XB was converted to wire spring relays in the 1950s and otherwise upgraded in the 1960s to serve exchanges with tens of thousands of lines. The final "5A Crossbar" variant, produced starting in 1972, was available only in sizes of 980 and 1960 lines, and generally delivered on one pallet, rather than assembled on site as usual for larger exchanges.