What is claimed is:1. A mobile storage system, comprising:a series of substantially parallel rails including a first end rail, a second end rail, and a series of intermediate rails located between the first and second end rails;a frame spanning across the rails and adapted to support one or more storage units;a series of mobile supports interconnected with the frame, wherein each mobile support is engageable with one of the rails for movably supporting the frame on the rail; anda drive arrangement, comprising a power source, a drive shaft driven by the power source, and a pair of drive units drivingly interconnected with the drive shaft, wherein each drive unit is engaged with one of the mobile supports inwardly of one of the first and second end rails.2. The mobile storage system of claim 1, wherein one or more of the intermediate rails are located between the rails with which the drive units are engaged, wherein the drive shaft spans across the intermediate rails.3. The mobile storage system of claim 2, wherein the pair of drive units are engaged with the mobile supports at a pair of spaced locations arranged symmetrically relative to a center of the frame.4. The mobile storage system of claim 1, wherein the drive shaft and each drive unit are coupled by a drive gear on the drive shaft having a first effective diameter and a driven gear on the drive unit having a second effective diameter substantially greater than the first effective diameter, wherein the drive units are operated in response to rotation of the drive shaft while minimizing torsional twisting of the drive shaft.5. The mobile storage system of claim 3, further comprising a pair of sprockets attached to the drive shaft, wherein each sprocket is engaged with a drive chain forming a part of the drive unit.6. The mobile storage system of claim 4, wherein each drive unit includes a pair of wheels engageable with one of the rails, and wherein the drive chain is drivingly engaged with each of the wheels for imparting movement to the wheels in response to rotation of the drive shaft.7. A method of driving a carriage for a mobile storage system which includes a series of substantially parallel rails including a first end rail, a second end rail, and a series of intermediate rails located between the first and second end rails, and wherein the carriage includes a frame spanning across the rails and adapted to support one or more storage units, and a series of mobile supports interconnected with the frame, wherein each mobile support is engageable with one of the rails for movably supporting the frame on the rail, comprising the step of driving the carriage on the rails by operation of a pair of drive units, wherein each drive unit is drivingly engaged with one of the mobile supports located inwardly of one of the first and second end rails, wherein the pair of drive units are located so as to be symmetrical relative to a center of the frame, to reduce skewing of the carriage upon operation of the drive units.