Effectively a 3-car version of the Class 317, 21 of these British Rail Mark 3-based units were built by BREL York works between 1985–1986 to replace the elderly Class 101, Class 107, Class 120 and Class 126 diesel multiple units (DMUs) which had worked the Glasgow South Western sector for nearly 30 years. The technical description of the units are DTSO+MSO+DTSO, consisting of a central motor car (with a roof mounted Stone Faiveley AMBR pantograph and four Brush TM2141 traction motors located under the floor within both bogies (two motors per bogie)) with a driving trailer at either end. The units run on the standard 25 kV AC overhead line system, and are standard class throughout.