Why were the class 27 built?

[randyrippley]

When the class 27 started production the class 33 were already being built as a class 26 development, and the class 25 were also being built.
It's always struck me that it would have made more sense to have built the 27s as the higher powered 33 given that many of them initially went to freight duties on the ER and LMR. If more steam heated type 2s had been needed then boilers could have fitted to more class 25s. Would have eliminated a complete class and helped rationalise spares stocks, while giving an uprate in power.

So why were the 27s built? Was it the old problem of Crompton-Parkinson not being able to hit delivery targets?

 

[Magdalia]

When the class 27 started production the class 33 were already being built as a class 26 development,

The BRCW Type 3 was not a development of the BRCW Type 2. It was a separate design to meet the Southern Region's different specification, including air brakes and ETH.

it would have made more sense to have built the 27s as the higher powered 33 given that many of them initially went to freight duties on the ER and LMR.

The BRCW/GEC Type 2s were not ER locos. They were ordered for area modernisation schemes in the ScR, NER and LMR. All required a loco with an 18 ton axle load which the BRCW Type 3 did not meet, even without a steam heat boiler. The NER ordered locos without a steam heat boiler, these were D5370-78. The LMR locos were used on passenger trains when new, notably boat trains between St Pancras and Tilbury.

If more steam heated type 2s had been needed then boilers could have fitted to more class 25s.

The first BR Type 2s built without boilers were D5151-D5175 for the NER, built at Darlington. I think these were the only BR Type 2 order built without boilers before the last BRCW Type 2 was completed.

At the time there was no spare capacity at other BR works to build more type 2s: Crewe and Derby were building Type 4s.

 

[McRhu]

The Class 27s were built to provide the Western Highlands with the finest specimen of poised and elegant loco-hood ever to grace the noble rails of Scotland.