The class was designed at Doncaster which was also responsible for building 25 of the 115-strong class. The remaining 90 were split between Horwich and Derby Works. The last in the series, No.76114, was also the final steam engine to be constructed at the ‘Plant’ as Doncaster works was known. The Standard Four Mogul was essentially a standardised version of the LMS Ivatt Class 4, and was primarily intended for freight use. Although a BR Standard the class did not have the same design of wheels as the Swindon-built 82XXX and 77XXX Class 3 engines which also had 5-foot-3-inch (1.600 m) driving wheels, yet all three locomotive classes share the same cylinder casting. The cylinder covers of engines built early in the programme of construction were fitted with “screw-in” type pressure relief valves. From September 1955 revised cylinder covers were introduced for renewals incorporating “bolt-on” type pressure relief valves.
An axle-loading of only 16 tons 15 cwt meant their route availability was virtually unrestricted. Batches were allocated to every BR region except the Western.
When new, 35 were sent to the Scottish Region where they worked over the bleakly dramatic Waverley route between Carlisle and Hawick. Others appeared on the ‘Port Road’ from Dumfries to Stranraer. The Scottish examples were mainly concentrated in Ayrshire and around Glasgow, and at one time Corkerhill depot was home to ten of the class. Five though found their way to Aberdeen and three to Thornton in Fife
The Southern moguls (originally 37) were also concentrated in one area around Eastleigh, Southampton and Bournemouth. They were used on the cross-country services between Portsmouth, Salisbury and Cardiff, Reading to Redhill, Brighton to Bournemouth and over the Swanage branch. Their most celebrated duty was the London Waterloo to Lymington boat train. However this had nothing to do with the engines’ capabilities; it was simply that among tender engines only a 2-6-0 or 4-4-0 could fit on the turntable at Brockenhurst. In the last few years of steam operation on the ‘Southern’ a few examples were allocated to Guildford shed before moving on to Feltham shed in S.W. London.
All 17 locomotives equipped with the BR1B high-sided tender were allocated to the Southern Region. Oddly the BR1B tender had a higher axle load than the locomotives.
At first the North Eastern Region scattered its 13-strong allocation far and wide: Darlington, Gateshead, Hull, Sunderland and York. Later all were concentrated at either Kirkby Stephen or West Auckland to work over the Stainmore route whose viaducts were not only spectacular but had severe weight restrictions. Like their small cousins the 2MT 2-6-0 class the Moguls were ideal for working the line. They worked coal trains as well as passenger services and were a regular choice for excursions from Tyneside to the Lancashire coast resorts.
Apart from a pair allocated to Leicester most of the London Midland Region’s batch of 15 spent their working lives in the north west around what is now Lancashire and Merseyside.
The Eastern Region divided its 15 between two London depots. Five went to Stratford on the ex-Great Eastern section, and the remainder to the one-time Great Central depot at Neasden. Made redundant by dieselisation the Stratford engines were transferred to the Southern and arrived at Brighton. The Neasden engines also in due course departed the capital, in this case for Chester and ex-Cambrian Railways territory. One Eastern locomotive was 76034 which was fitted with a tablet catcher for running over the M&GN lines in East Anglia.