The original underframe, bogies, traction motors and brake system will be retained and overhauled. The new prime mover required substantial alterations to the cab, carbody, and underframe. Sides consist of fabricated frames with steel panelling welded to the underframe. The trailers were constructed principally of aluminium alloy on a steel underframe. The coaches were of integral aluminium alloy construction based on a steel underframe. In earlier versions, the EMD chassis / underframe was used and was visible. The gearbox is mounted on a separate underframe; all its shafts run on ball bearings. All except the snowplow were on the heavy 30-foot underframe. The locomotive's underframe, body, and engine are brand new. The Mark II was also available as a bus underframe for other coachbuilders to body. The restored van will have a steel underframe clad in wood. A National bus-type body was placed on the underframe from Mk1 BCK coach number 21234. It has different underframe for handling larger buffing loads. This was the Royal Tiger underframe and the Roe Doyen body. Those cars need a stronger frame, hence the fishbelly underframe.