The class ran to four members: source: BritishSteam[1] The locomotives were large and visually very impressive, but they were not successful and never really got established in traffic.
The valve gear was insufficiently robust and the valve spindle guides (originally cast iron) had to be replaced in cast steel, but this did not address the fundamental shortcomings of the design and the performance of these large locomotives remained woeful.
[2] By early 1922 management decided things could not be left alone and in April of that year No.s 957 and 958 were rebuilt to the strange arrangement of Walschaerts valve gears for the outside cylinders and Stephenson link motion for the inside one.
959 was given a modified version of the standard Caledonian equipment but with dashpots added to absorb stress that was thought to exist at some points.
Experienced locomotive engineers have expressed astonishment and disbelief at the "cacophony" that the mixed valve gear gave rise to, and many have stated that the whole idea was a disaster.