GWR chief mechanical engineer G J Churchward created a range of standard locomotive designs, using as many common sizes and parts as practical. For branch line and suburban work tank engines which were able to travel just as well in reverse as forwards were desirable and two sizes of 2-6-2 tank engines were created. The 45xx class were fitted with 4'7" driving wheels for branch line service, while 5101 class was in effect a tank engine version of the 43xx general purpose 2-6-0 mogul. Naturally the two classes quickly became known as the 'small' and 'large' prairies.
Equipped with 5'8" driving wheels, large 18x30in cylinders and a free-steaming boiler only slightly smaller than the moguls the 5101 class were highly capable engines, easily able to haul heavy suburban passenger trains and cross-country stopping services over secondary routes. These powerful tank engines were also used extensively as pilot and banking engines on the GWRs more challenging grades in South Devon, the Midlands and the Severn Tunnel.
Several sub-classes were built, the most numerous being the 61xx class which had a higher boiler working pressure (225psi vs 200psi for the 5101 and 43xx), boosting the tractive effort past that of many class 5 4-6-0 engines. These locomotives were originally built for the heavily loaded and tightly timed London outer suburban services, though later many migrated to 'country' sheds working branch lines where crews appreciated the boilers' reserve of power, even against the 5101.