Middle Atlantic R412-RR38 The R412-RR38 Rack Rails for 38U R412 R4 Series Racks (Pair) from Middle Atlantic is set of rack rails designed for use with 38U R4 Series Four-Post Open-Frame Rack. This unit consists of one pair of rack rails. The rails are constructed from 3/16 inches thick extruded aluminum and feature 12-24 threads. Use this unit to complement existing rails in your D4 series rack or replace damaged rails.