Originally built as a
petrol electric rail motor (PERM), they were the longest-lived rail motor on the Victorian Railways, with the first entering service in 1928 and the last being withdrawn in 1991. The rail motor, also known as the
railcar, was a standard product of the US Electro-Motive Corporation (a predecessor of
Electro-Motive Diesel) and built between 1924 and 1932, albeit to a smaller
loading gauge and wider
track gauge. The first was imported in 1927, assembled at
Newport Workshops, and placed in service in 1928. The bodies of the remaining nine were constructed at Newport Workshops using imported equipment and electrical components, and placed in service between 1930 and 1931. In 1976, RM 56 was fitted with a new seating arrangement, including rotating seats, using components from the
Z carriage fleet. Capacity was reduced to 34 passengers, because the new seating arrangement was two either side of the central aisle rather than the previous two and three. The first-class end of the vehicle was moved towards the centre, closer to the engine, but providing a smoother ride by averaging the suspension of both bogies rather than just one. The trial arrangement was used on the
Yarrawonga line, but patronage did not increase sufficiently to justify the retention of that service. Following the changes to 56RM, 55RM and 61RM were extensively modified in the late 1970s, with the engines relocated, the body extended, and a new seating arrangement provided. The most obvious external difference was the fitting of aluminium-framed windows on the driver's cabin, and porthole windows for the engine room. Those "Super DERMs" were a familiar sight on the adjoining
Mornington and
Stony Point lines prior to the early 1980s. ==Class summary==