Middle Atlantic PROMRK-RR37 The Middle Atlantic MRK Cage-Nut Style Rackrail Kit PROMRK-RR37 is designed as an alternative to the standard 11 gauge, 10-32 threaded rackrails included in the MRK Series of racks. Simply remove the standard rackrails and replace them with this kit to fit your needs. Cage nuts and rack screws must be purchased separately. This model is made specifically for MRK 37U racks.