The NZR RM classClayton steam railcar was a unique railcar that operated on New Zealand's national rail network and one of only two steam railcars to operate in New Zealand - the other being 1925's RM class Sentinel-Cammell. Built in 1926 by Clayton Carriage and Wagon of Lincoln, England and assembled at the New Zealand Railways Department's (NZR) workshops in Petone, the railcar could seat up to 52 people and its steam boiler could generate a pressure of 275 psi. It could be driven from either end and was capable of hauling a wagon or two of freight, and its airy, open design proved popular with passengers. However, it was not popular with crews or mechanics. Before it even commenced revenue operations, a heavier firebox and larger boiler had to be installed, and its poor reliability necessitated regular repairs. Due to these issues, no additional examples of the railcar were built.
After its assembly and improvements were completed, the railcar was transferred to the South Island and took over passenger duties on the Kurow Branch, running from Kurow to Oamaru and return six days a week. Previously, the line's passenger services had been worked by mixed trains that carried both passengers and goods and thus would regularly stop for extended periods to load and unload freight, and the steam railcar proved to be a vast improvement, completing the journey an hour quicker, in 1 hour 45 minutes. It could maintain a speed of 30 mph (48 km/h) on straight, flat track, but when presented with steep grades or sharp curves, its speed would drop to 6–14 mph (9.7–22.5 km/h). One quirk of its operations was that farmers' dogs had to be carried in dog boxes for the duration of the trip rather than lying at their master's feet; as the railcar operated on a rural branch line, this policy was not greeted with enthusiasm.
On 10 November 1928, a regular locomotive-hauled passenger train replaced the railcar, but this was not the end of its life. It was assigned to run various services in Otago and Southland for a number of years, but after eleven total years of operation, it was withdrawn. Railcar technology was improving (the Vulcan railcars were ordered not long after the Clayton retired) and there was little need or economic justification for an older, somewhat unreliable steam-powered railcar that was not suitable for the rugged terrain that typified many lines in New Zealand. It was withdrawn and did not survive to be preserved. Its sandbox was appropriated for use as a back sand gear on AB 810.