British United Traction was a joint venture between Leyland and AEC, set up after the war to produce road vehicles. With the imminent modernisation of the railways they set up a rail section inn 1953 and eventually supplied around 75% of all the DMU engines and control equipment in the UK, and also had extensive sales abroad.
They were not so succesful with their complete rail vehicles in this country. Some were built under its associated companies — Park Royal was a sibling company to AEC, both being part of the Associated Commercial Vehicles (ACV) group.
Both companies had previous railcar experience, AEC most noticably with the GWR railcars and Leyland with three it built for the LMS. A detailed history of the company can be found in the publicity booklet.