A rail assembly includes an outer rail, a middle rail, an inner rail, and a retrieving unit. The middle rail has a receiving space defined in an underside thereof, such that a length and a moving range of the middle rail are increased. A guiding member is disposed in the inner rail and temporarily engages with a clip to connect the inner rail and a moving part. At least one spring is disposed between a fixing part and the moving part. Two receiving slots are respectively disposed between the fixing part and the moving part with the outer rail, such that the inner rail spans across the receiving slots to reach the closed end of the outer rail space for increasing an overall spread length of the rail assembly.