[3] Following the success of the K class in the North Island, there was a need for similar locomotives to operate the Midland Line, primarily between Springfield and Arthur's Pass.

The ACFI equipment's aesthetic appearance had been criticised, hence the KB and contemporary KA classes were both fitted with shrouding to obscure it.

[3] The use of the booster, first proposed for the K class in 1932, was mainly due to steeper gradients and heavy freight trains on the Midland Line, especially Cass bank.

[2] As intended, the KB class were based solely in the South Island and performed most of their work hauling freight trains on the Midland Line.

While the aesthetic shrouding cleaned up the appearance of the locomotives, it was open at the top and gathered soot and dust which affected the working environment in the cab.

[2] This left only three KB's in service (965, 968 and 970), and of those, only 968 and 970 were still used regularly while 965 was kept as a standby spare locomotive from July 1968 onwards; with the remaining two being withdrawn in March 1969.

[5] KB 968 was supposed to be cut up the week following its last excursion, but a collection on the return journey was sufficient to temporarily save the locomotive.