Abstract: A movable carriage assembly for a door track or window track is adapted to support a door or window. The carriage assembly includes a lower carriage removably joined to an upper carriage, forming an elongate gap there between which is constructed and adapted to provide a sliding surface element for a track dust cover. When the carriage assembly moves along the track, the dust cover moves through the gap of the carriage assembly and over the sliding surface element whereby the dust cover is caused to disengage from the track and re-engage with track after the carriage assembly has passed. There is also disclosed a method of installing the carriage assembly in a window or door opening for a door or window.