Mr. Changalwala Srinivasa Rao has given a baseless information. Side Middle Berth came into existence only because of Garib Rath Trains. In addition to GRs, this concept was applied even to normal Sleeper and 3AC classes which is now in the process of reversa.
Yet, in GR trains, it has been since the inception and will remain for the time being as no modificatory resolution has come up so far.