The '1001' class was one of Bouch's "long boiler" 0-6-0 designs

These locomotives had the entire weight of the engine on the six driving wheels. The boiler was notably long, but the fire grate was small - especially when compared to later design principles. This basic design concept was perpetuated on the Stockton & Darlington for a long time, and was decidedly old-fashioned in its later years. Despite this, the design actually suited typical S&D mineral workings. These workings were generally short and involved lots of waiting for signals or loading. The waiting allowed the locomotive time to build up pressure in its large boiler. The short workings meant that boiler pressure could be sustained long enough for the working to be completed. Hence the small grate did not provide a significant limitation, but actually contributed to economic running.

 

 

No. 1275 was the only '1001' locomotive to survive into LNER ownership. Due to the wide variability in the '1001' locomotives, this page will concentrate on No. 1275. No. 1275 was built as works no. 707 from an order placed with Dübs in April/May 1873. Delivery was in May 1874.
A McDonnell-pattern boiler was fitted in August 1883. This boiler was 4ft 1in diameter and had 190 tubes. This was followed by a standard W.Worsdell boiler in June 1896. This new boiler had wider water spaces than the earlier McDonnell boiler. When No.1275 entered LNER ownership, it was still fitted with a Worsdell boiler, although this was a secondhand one that was fitted in July 1906 (from No. 1014).

 

The Stockton & Darlington used the '1001' locomotives for the majority of its mineral workings for a very long time. By the 1870s, when No. 1275 was introduced, they saw extensive use hauling Durham coke to Barrow via Stainmore Summit. They also hauled haematite ore from Whitehaven to Kirkby Stephen via the Cockermouth Keswick & Penrith line and the Eden Valley.
By 1906, many were still being used on the steep lines in the Loftus area. Although they were still held in very high esteem, they were used for less arduous work during their later years. A clear exception was the exposed Rosedale branch. Here, a number of '1001's were used to haul ironstone to the head of the Ingleby incline. Nos. 1255 and 1286 survived on this duty until 1921 when they were replaced by J24s. In these later years, No. 1275 was allocated to Malton and was used to haul local goods services to Whitby. The Malton shed is noted for keeping No. 1275 in a very clean condition, and O.S. Nock described his first-hand experience of the pride that the Malton enginemen had for what was already an antiquarian engine.
No. 1275 entered into LNER ownership with an official mileage of 908,984

My thanks to Wikipedia for the above