GWR 64xx/74xx 0-6-0 Pannier Tanks

Designed for auto train services
The Collett design 64xx class 0-6-0 pannier tanks were designed to operate with up to 4 auto trailer coaches as push-pull 'auto trains'. The 6-coupled pannier design was developed through trials with older pannier tank locomotives to power more heavily loaded trains than the 517 (and later 48xx/14xx) 0-4-2 tank engines could manage. The 64xx class was essentially a small-wheeled (4'7" wheels) version of the 54xx class (5'2" wheels) which were built for suburban passenger 'auto trains' in the London division. The smaller wheel diameter made the 64xx better suited for service over the hillier routes in south Devon and south Wales.
The 74xx class was almost identical except for the fitting of a boiler with a higher working pressure and omission of the auto train control gear. This class was used where a small, modern engine was desirable for longer branch lines where hauled stock was used and at locations where principally goods working was required. 74s were deployed to west and mid Wales branch lines, including the Bala to Blaenau line and around the Black Country sheds.
Three of the 64xx class have been preserved, including 6412 which was notable for it's appearance at many branch lines' 'last day' in the late 50s & early 60s before going into preservation.