Coaches 1CM and 2CM entered service in 1921, with a capacity of 25 tons each. In 1923 they were joined by a third vehicle, 3CM. This was partially paid for by the Electrical Engineering Branch, with a modification to the design by addition of a centre cupola for viewing of overhead wires. This van was run with the normal fleet, but was made available for overhead wire inspection if and when required. CMs 4 and 5 entered service in 1925 and 1926 respectively, both using the new arch/curved roof style between their two cupolas.