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Boulton's Siding was a British locomotive-hire business owned by Isaac Watt Boulton and situated alongside the Oldham branch of the Manchester, Sheffield and Lincolnshire Railway at Ashton-under-Lyne. It operated from 1864 to 1898.
In the 1860s, established railway companies were replacing their small 2-2-0 and 0-4-0 steam locomotives with larger, more modern machines. Boulton took advantage of this by buying up some of the old locomotives, modifying them in his workshop, and hiring them out to contractors for use on civil engineering projects, including the building of new railways.
The usual modifications were to fit smaller driving wheels (to increase tractive effort) and to convert tender locomotives to tank locomotives.
1881 Sale of engines, machinery, plant and tools etc. at Boulton's Siding Locomotive Depot and Engine and Boiler Works and Portland Street Engine Works, Ashton-under-Lyne 
By the 1880s, the hire business was declining so Mr Boulton started building new locomotives. He used his own patent design of boiler which had a single large flue with cross water-tubes. This was, essentially, a horizontal predecessor of the vertical water-tube boilers later used in Sentinel Steam Waggons. Although Boulton's boilers steamed well, they were difficult to repair so they never became popular. The business declined during the 1890s and closed in 1898.