Four locomotives were built in 1907/1908 for the Great Central Railway. They were fitted with 3 cylinders. This gave a more even torque than with a 2-cylinder locomotive and reduced the risk of wheelslip under heavy load. All four passed into London and North Eastern Railway ownership at the 1923 grouping.
These were powerful locomotives but even more power was required so, in 1930, one locomotive was fitted with a superheater and a booster engine and classified S1/2. Two new locomotives (with superheaters and boosters) were built by the LNER in 1932 and classified S1/3. The remaining (non-booster) locomotives were classified S1/1 and were also fitted with superheaters. All the boosters were removed in 1943.
All 6 locomotives passed into British Railways ownership in 1948 and were numbered 69900-69905.
For terminology, see Steam locomotive components
Ian Allan ABC of British Railways Locomotives, 1948 edition, part 4, page 56