Using differential gearing to transmit the power, it was built as a 4-8-4 with the coupling rods connecting the centre four pairs of driving wheels. The coupling rods between the innermost axles were later removed, but as the four axles were driven by a single gearbox, it remained a 4-8-4. At it was the most powerful of BR's non-steam locomotives at the time. From 1951 it worked the expresses from Manchester to London, proving some 25% more powerful than 5XP 4-6-0s. While the mechanical transmission made it much lighter than the diesel-electric locomotives, its complicated mechanism made it difficult to maintain (a working model of the transmission is on display at the National Railway Museum, York).