The 4-post Rack Replacement Rail Kit contains a pair of #12-24 threaded replacement equipment rails for 45RU R4P style 4-post racks. The rails can be used to change the rack from cage nut to threaded or can be used to add an additional set of rails to an existing rack for additional component mounting options. The steel rails are black with RU numbers printed in white. These rails are only compatible with R4P style 4-post racks.