Abstract: The disclosure herein describes a modular rail for use in the assembly of a mobile filing and storage system that includes cabinets each equipped with a roller carriage in their lower part. The rail is a longitudinal metallic extrusion that has a cross-section that defines a first open side section adapted to receive therein one side of a floor structure over which the system is mounted, an intermediate second section adapted to receive thereon roller members of the system carriage and a third section that consists of a first groove adapted to receive levelling bolts for vertically adjusting the extrusion relative to a floor on which the system is supported and a second groove adapted to removably receive a cover adapted to extend over the first groove. The cover is removed when the rail is used for expanding the system.