The following answer is taken from Wardale's response to an unpublished letter from Bryan Attewell written in response to Wardale's two-part article "Steaming into the Future" published in Steam Railway magazines #272 and #273 (see 'Articles and Letters' page of this website):
"Regarding the designations "mixed traffic" and "express" we should not forget that for the larger BR Standards (i.e. those with 6' 2" coupled wheels) "mixed traffic" meant what it said, and certainly included express duties (the Britannias were used on little else). It was the improvement in front-end design which allowed latter-day steam locomotives to become 'maids of all work' - a colleague during my BR days was only partly joking when remarking that for the schedules of the late 1950's only two classes were required - 0-6-0 diesel shunters and 9F's. Even with 1950's design 6' 2" wheels were no barrier to 90 mph running, and with the 5AT they will be no barrier to 100+ mph."