Built in 1986 – 1987 by Andrew Barclay (later Hunslet Barclay) of Kilmarnock, utilising a modified bus body design by Alexander, these units were part of the re-equipping of Regional Railways in 1985 - 1986. They have been extensively re-worked since build, receiving new engines, transmissions, brakes, doors, seats and fire systems. They now have Cummins L10 engines driving through a Voith transmission, recognised as the most dependable power train on any DMUs. The units are shorter than normal, being only 15.55 m long, and only have 3 doors per side. The space saved is used to accommodate a toilet in one vehicle and more seats in the other. The original 3 x 2 seating has been replaced by 2 x 2, offering greater comfort and reducing dwell times at busy times. Construction is of aluminium body panels resiliently mounted on a steel underframe, a combination of bus and rail engineering. The units have no bogies, being based on a 4-wheel platform. Couplers are common with the Class 15X “sprinter” fleet, but would need a minor electrical modification for Class 17X compatibility.