Complex sentence: The stone viaduct was part of the Kilmarnock and Troon Railway, it has four arches with sandstone ashlar facings rounded cutwaters which were extended later to form semi-circular buttresses.
Split into simpler sentences: The stone viaduct was part of the Kilmarnock and Troon Railway. It has four arches with sandstone ashlar facings and rounded cutwaters: these were later extended to form semi-circular buttresses.