Steam train failed to proceed.

[David Benton]

It appears they were retrofitted later.
https://en.wikipedia.org/wiki/NZR_A_class_(1906" onclick="window.open(this.href);return false;)

[johnthefireman]

Not an uncommon procedure. Steam locomotives are long-lived beasts which represented a considerable capital investment, so they were constantly modified and rebuilt and the end product several decades later often bore little more than a superficial resemblance to the original.