Holden , but were much rebuilt, resulting in several subclasses. Seventy-one S69 locomotives were built between and and numbered β At the time of their introduction, the "Claud Hamilton" s were becoming outclassed on the heaviest express. Although an enlarged design was mooted, [ 3 ] it was realised that any such design would have too high an axle load for the tracks of the Great Eastern Railway , which had a relatively low restriction.
Another design constraint was the short turntables used at the time. This meant that a design was decided upon, although the design was relatively short compared to similar designs introduced at the same time. The first locomotive, numbered , was delivered to Ipswich shed in November , and construction continued at Stratford Works up to number which was delivered, again to Ipswich shed, in June These were delivered from June to April during which time construction continued at Stratford Works with numbers to also being delivered in After the grouping the LNER ordered a further batch of 10 locomotives from Beyer Peacock of Manchester, and these were delivered with running numbers to to Gorton shed.
This final batch brought the number of locomotives constructed to Seventy were still in service at the grouping , the LNER adding to the numbers of nearly all the ex-Great Eastern locomotives, including the Class S69 locomotives. A further ten were ordered in to ease a power shortage caused by the stalled development on a new class of locomotives , and the cancellation of the planned suburban T tank locomotive due to the adverse press publicity caused by the Sevenoaks derailment of All the B12 locomotives were fitted with vacuum ejectors between and the batch had them from new.
Fifty-five locomotives were fitted with ACFI feedwater heaters between and , but these were removed between and The poppet valves were not a great success and they all reverted to or were converted to piston valve engines between and Starting in , a programme began of rebuilding the B12 locomotives with larger diameter boilers.
As the Scottish locomotives also required new boilers, a new design was started in , based on the old design but with a round-topped firebox and other detail changes.
