 The basic design of the " Britannias " owed much to LMS building practices , especially when considering Riddles ' previous career with the said railway . However , in keeping with the necessity to follow best practice in creating standardised steam locomotives , they utilised a variation of both boiler and trailing wheel of the Merchant Navy Class , while weight was kept within the margins laid down by the Light Pacifics , all of which were designed by Oliver Bulleid . The firebox was also similar in having a rocking grate , which allowed the fire to be rebuilt without stopping the locomotive , removing both ash and clinker on the move ; but unlike the SR Pacifics , the inner firebox was constructed of copper instead of steel , and lacked thermic syphons . A self @-@ cleaning smokebox was used , which enabled ash to flow into the atmosphere , reducing the workload of the engine cleaner at the end of a working day . A single chimney was placed on top of the smokebox , which was unusual for a " Pacific " type of locomotive . This was because the blastpipe was designed by <unk> Ell at Swindon Works , who claimed that " better results could be obtained from a well @-@ designed single chimney than some of the previous double chimney arrangements " . The " Britannias " had 6 ft 2 in ( 1 @.@ 88 m ) driving wheels , a compromise that took into account the intended mixed @-@ traffic role they were designed for . This meant that they were large enough for sustained fast running with heavy passenger trains , yet small enough to allow them to undertake more mundane tasks such as freight haulage . 
