The RM class was the classification used by the New Zealand Railways Department (NZR) and its successors gave to most railcars and railbuses that have operated on New Zealand's national rail network. "RM" stands for Rail Motor which was the common name at the turn of the 20th century for what became known in New Zealand as railcars. As many types of railcars are operated, class names have been given to each railcar type to differentiate them from others.

A mixed train or mixed consist is a train that contains both passenger and freight cars or wagons. Although common in the early days of railways, by the 20th century they were largely confined to branch lines with little traffic. Typically, service was slower, because mixed trains usually involved the shunting (switching) of rolling stock at stops along the way. However, some earlier passenger expresses, which also hauled time-sensitive freight in covered goods wagons (boxcars), would now be termed mixed trains. Generally, toward the end of the mixed train era, shunting at intermediate stops had significantly diminished. Most railway passenger and freight services are now administered separately.