In 1981 Comeng, Dandenong were building new suburban sets for the system. The railways were interested in using the Harris cars shells fitted with new equipment in an effort to reduce costs. The Harris cars were originally insulated with asbestos which was proving to be hazardous in the workforce.
Sixteen cars were eventually rebuilt and operated as four "four car" sets. They were expected to run as eight car sets on the Belgrave/Lilydale lines. However with the increase in tare weight in rebuilding and with the same traction motors used, they were underpowered. They saw service only on the Sandringham/St Kilda/Port Melbourne lines which were relatively flat.
The M cars were numbered 901 - 908 when delivered and were converted from standard Harris _M__ cars. Several months later they were renumbered to 601 - 608. However this number group conflicted with the new Comeng car numbers and they were renumbered back to 900's several months later.