With the introduction of the new Class 800 and 802 IEP units on the Great Western Mainline GWR cascaded the remaining IC125 sets to rural routes shortening them to 4 passenger coach 'Castle' sets. This HST 'Castle' Train Pack contains two particular power cars.
Power Car 43004 entered service in September 1976 and was one of the prototypes for the Brush MTU engine upgrade, this occurring between September 2004 and June 2005. As a result, it did not receive a full overhaul as other members of the Class did, and so returned to Brush to receive Cab upgrades in November 2007. Its counterpart, 43155 entered service in January 1981 and was overhauled by Brush between April and October 2006.