Abstract: In a document storage and retrieval system of the type including at least one storage rack having a plurality of individual storage compartments, each of the compartments normally having positioned therein a cartridge holding a plurality of cards or the like, at least a first one of the compartments being empty and functioning as an access port for feeding cartridges into and removing cartridges from the storage rack, and a carriage assembly which travels on one side of the rack for transferring cartridges between the access port and the other storage compartments, the carriage assembly including a transport mechanism for moving cartridges thereon to and thereoff of, there is disclosed an improved transport and gate assembly positionable in a single empty storage compartment immediately below the access port which acts as a stop and cartridge presence sensor for blocking movement of a cartridge through the access port on to or off of the carriage assembly and which selectively retracts and allows movement of