Tripp Lite Universal Heavy Duty Rack Enclosure Server Cabinet Shelf Kit, Black

I returned another manufacturer's rails that were claimed 250 lb capacity, but they were very flimsy and flexed under the weight of several 40-70 lbs test equipments. The test equipment cost over $100K and no way would I trust those other rails. Replaced with these Tripplite 4POSTRAILKITHD rails and WOW! What a difference! They are rated for 500 lbs and built like the proverbial battleship. Thick 12 gauge (0.11") zinc-plated steel, with "rolled" edges, so they are stiff in all directions. The TrippLite rails mount _behind_ your rack's vertical columns, therefore the equipment will mount flush against the rack's columns. In hindsight, perhaps the less expensive Tripp Lite 4POSTRAILKIT standard-duty rails would also have worked OK -- Tripp Lite seems very conservative with the load ratings -- but given the very high price of the test equipment installed in the rack, there was no reason to take a chance. ADDED: The flat "shelf" portion of these heavy duty rails is 1.38 inch wide, whereas it's only 0.88 inch on the standard duty TrippLite model 4POSTRAILKIT. If the equipment has a narrow chassis width, it might actually fall through (!) the standard rails, or be inadequately supported. The heavy duty rails provide a wider "shelf" under the equipment's chassis.