At first sight the ‘Granges‘ might appear to be nothing more than smaller-wheeled versions of the Great Western’s ‘Hall‘ class. They utilised the same boiler as the ‘Halls’ and their pistons had the same diameter and stroke. However, there were subtle differences between the two that would set the ‘Granges’ apart from the ‘Halls’ in the minds of many footplatemen. The ‘Granges’ became noted for the readiness with which their boilers raised steam; superbly free running and impressive delivery of power when attacking steep, adverse gradients with heavy loads. It was precisely because of these properties that the Great Western allocated so many of them to the arduous duty of piloting heavy passenger and freight trains over the notoriously steep and twisting South Devon banks.
The ‘edge‘ that the ‘Granges‘ had is undisputed but can only partly be explained in rational engineering terms. The critical difference between the ‘Granges’ and the ‘Halls’ lay in the dimensions of two chambers at the front end of the locomotive’s steam circuit, namely the steam chest and the steam ports. Both were larger on the ‘Granges’ than the ‘Halls’. The greater steam port volume is said to give better cushioning of the piston at the end of each power stroke. It is believed that this accounted for the marked absence of fore-and-aft surging on the ‘Granges’, but which is very apparent when riding behind a ‘Hall’ to this day! Despite great efforts to make steam locomotive development a more theoretical and less empirical discipline, it always relied heavily on experimental observation as a means of proving a design. The truth is that while the characteristics of a locomotive could be observed and measured in great detail, it was not always possible to explain why they performed as they did. The legendary free steaming of the ‘Granges’ was just such an enigma. They were fitted with exactly the same boilers as the ‘Halls’ (in fact, boilers were frequently swapped between the two types). The draughting arrangements in the smokebox, critical to a locomotive’s steaming characteristics, were identical. Yet the truth is that time and time again the ‘Granges’ steamed noticeably more freely, a quality much appreciated by hard pressed footplate crews. It is hardly surprising that the ‘Granges’ came to be regarded as ‘The Enginemen’s Engine‘.