Supporting device for rail member

A supporting device for a rail member comprising:

• a first supporting frame having a first longitudinal body, a pair of first transverse parts being arranged on the first longitudinal body, each of the first transverse parts having a first supporting feature;
• a second supporting frame movably attached to the first supporting frame, the second supporting frame having a second longitudinal body, a pair of second transverse parts being arranged on the second longitudinal body, each of the second transverse parts having a second supporting feature;
• and
• a third supporting frame movably attached to the second supporting frame, the third supporting frame having a third longitudinal body;
• wherein the third supporting frame is fixedly attached to the rail member, the first supporting frame and the second supporting frame are both movable relative to the rail member;
• wherein the first supporting feature of the first supporting frame is configured to support the second supporting frame, and the second supporting feature of the second supporting frame is configured to support the third supporting frame;
• wherein a first bracket is arranged on one of the rail member and the third supporting frame, and a second bracket is arranged on one of the first supporting frame and the second supporting frame, each of the first and second brackets comprises a side wall and an end wall substantially perpendicularly connected to the side wall.