Whatever its fame and history, a locomotive can also be seen as a collection of skilfully designed, manufactured and maintained components, with the locomotives frames often considered the source of its identify, and other parts frequently swapped between different locomotives within the same type and sometimes between types. As a result of this rotation and despite the fact there were 35 A4s, there is a remarkable circularity between the six surviving A4s, and three of the surviving A4 boilers.
When steam traction was in daily use on Britain’s railways, parts were frequently swapped as locomotives were overhauled. Stocks of overhauled spares were kept to speed the passage of a locomotive through the workshops, in order to prevent work on a particular component slowing the overhaul of the locomotive as a whole. As a result locomotives frequently carried parts that had been fitted new to other locomotives. Careful inspection of locomotives at York and Shildon will on occasions reveal parts stamped with numbers different to the locomotive to which they are now fitted.
The A4s were no exception to this rule, although there are some notable cases of normally exchanged components either serving with a single locomotive, or parts finding their way back to original locomotives. In particular, Bittern has spent its entire life paired with the same tender with which it was fitted new in December 1937. A new corridor tender body replaced the non-corridor original at its last overhaul in 2007. Dominion of Canada was fitted with four different tenders during its working life, but was paired with the same tender at both the start and end of its service, and is preserved with the same.
One of the most frequently swapped major components on a locomotive was the boiler. Overhauling a boiler would often necessitate major work that could not be completed in the same time span as a locomotive overhaul. At a General Overhaul when a boiler was lifted, it was normally quicker to refit a newly overhauled boiler than to wait for the removed boiler to be overhauled, especially so given the number of components to be removed before a boiler was lifted, or refitted after a boiler was installed.
To speed this process, and to replace boilers deemed beyond economic repair, a large number of spare boilers were built during the working lives of the A4s. The 35 A4s were fitted with ‘Diagram 107’ boilers. In total 88 of these boilers were built, although not all were in existence at any one time. During later years, these boilers were also fitted to some A3 locomotives, including most recently Flying Scotsman from 1978-2006. The final two batches of twenty-five new A4 boilers were commissioned in 1959-60. All of the surviving A4s carry these late-built boilers. Seemingly against statistical odds with a fleet of 34 locomotives in 1962 and an even larger pool of diagram 107 boilers available, there is a remarkable commonality between the preserved A4 locomotives and three of the surviving boilers. Between them, three of the surviving boilers have a BR service history solely on the 6 surviving A4s:
- 4468 Mallard today carries boiler 27965, fitted 1961 and previously fitted new to 60009 Union of South Africa in 1960.
- 4489 Dominion of Canada today carries boiler 27970, fitted in 1962 and previously fitted new to 60007 Sir Nigel Gresley in 1960.
- 60008 Dwight D Eisenhower today carries boiler 29335, fitted in 1962 and previously fitted new to 60019 Bittern in 1960.
The Great Goodbye at Shildon is therefore the last opportunity, certainly this generation, not only for 6 A4s to be assembled, but for a reunion of three Diagram 107 boilers (27965, 27970 and 29335) and the 6 locomotives that, between them, they were fitted to for their entire working lives.
The author acknowledges the excellent series of articles in the Bewilderment of Boilers series by Mel Haigh of the Sir Nigel Gresley Preservation Trust, and the boiler records on BRDatabase.info as the source of much of the data for this article.