A driving mechanism having a braking system, for use on a storage unit movable about guiding tracks, The driving mechanism comprises a frame rotatably carrying at least one driving wheel, which in turn rollably engages a floor-embedded guiding track, A toothed wheel rotatably carried by the frame is operatively connected to the driving wheel and rotates in synchronism therewith, and defines at least one first interlock member, The driving mechanism also comprises a gravity-biased second braking member defining at least one second interlock member, said second braking member movably mounted to the frame so as to be movable between a standby position in which the second interlock member clears the first interlock member, and a braking position in which the second interlock member is interlocked with the first interlock member to block rotation of the rotary first braking member and thus of the driving wheel, The second braking member is biased under the action of gravity towards the braking position when it is in its standby position, The driving mechanism further comprises a brake control member mounted to the frame, the brake control member being movable between a retaining position in which the brake control member engages and retains the second braking member in its standby position against the bias of gravity, and a release position in which it clears the second braking member to allow gravity-borne movement of the second braking member towards the braking position. The second braking member is free of any mechanical structural link with the brake control member.