In the early 20th century, NZR began experiments with railcars as an option to replace unprofitable regional locomotive-hauled carriage expresses and to provide an efficient passenger service on rural branch lines that were served solely by slow mixed trains that carried both goods and passengers. In 1925, a steam railcar was ordered from the England-based Sentinel Waggon Works of Shrewsbury and Metro Cammell of Birmingham, and when it entered revenue service, it was the first railcar to do so in the Auckland Region. It subsequently operated outside this region.
The railcar failed to be fast enough for the Melling Branch, so it was assigned to run a feeder service for the Night Limited express that ran between Wellington and Auckland. This service operated from Thames along the Thames Branch and met the express before returning to Thames. This service was not the sole domain of the Sentinel-Cammell steam railcars, however; it was sometimes a carriage train hauled by steam locomotives such as the UD class. In 1928 it survived a collision with cows.