Abstract: An elevated track network guides self-propelled guided vehicles (with containers thereon) from an entry location where objects enter the system, to a storage zone defined by sets of compartmentalized storage racks wherein containers filled with objects are temporarily stored, and from thence to an exit location where retrieved objects leave the system. Each set of storage racks comprise four racks and two aisles, with two of the racks arranged back-to-back between the aisles and the other two racks on opposite sides of the aisles. Each storage rack comprises storage compartments arranged in horizontal rows and vertical columns. A track in the track network extends through and along an intermediate row in one of the back-to-back racks (pre-empting the compartments in that row). Each aisle accommodates a self-propelled aisle crane which services the racks on both sides of the aisle.