A single carriage is listed as suiting Cowdroy™ track: a 19mm twin steel ball carriage in a plastic body, also listed as replacing Trak™ carriages. Twin steel ball wheels in a plastic housing are a light duty pattern, and they are what gives out — the balls pick up grit and stiffen, the plastic body splays under load, and the door starts to bind or run unevenly in the track. With one part listed and no alternative sizes, measuring is the whole job. The 19mm in the title is the identifying dimension, taken across the wheel, and it is what separates this carriage from others that look near enough identical in similar track. Check the wheel diameter and the body width of the old carriage before ordering.