The present invention provides a rail system for a horizontally sliding door or window having a construction casing; and at least one slideable panel. The system includes, at least one track from which or on which the panel slides. Each track includes, at least two track segments whose length is shorter than the width of the slideable panel(s). The track further includes a track segment fastening mechanism for allowing the track segments to be removed and re-inserted into the construction casing; and a spacer disposed between adjacent track segments. The total length of each spacer and one of each of its adjacent track segments is greater than the width of the slideable panel(s).