Abstract: The present invention relates to an upper and a lower sliding mechanisms as well as an upper rail and a lower rail structures for showcases, wherein the lower rail structure has two rail grooves with wider and shallower profile than usual rail for the same purposes and on which wheels mounted on the lower sliding mechanism can freely roll; and the lower sliding mechanism is equipped with a wheel lock having a stop section which may be extended outward and retracted inward by using a key to respectively form a stable locking condition and an unlocking condition for the showcases. The upper rail structure has a substantially M-shaped cross section, and two rail channels are formed at each side of a middle recess. In the upper sliding mechanism, two n-shaped insert channels are respectively disposed in the two rail channels to fitly receive glass sheets for showcases.