 This was the result of short-framed M7s having long frames substituted during overhaul in order to create room for the air compressor.Following the successful use of superheating on other Drummond classes, Robert Urie experimentally fitted a superheated boiler to No.126 in December 1920, together with an extended smokebox and larger cylinders.The additional weight of the new boiler raised the centre of gravity of the locomotive, thereby adding to problems of instability on faster main line trains, whilst simultaneously preventing its use on many branch lines.