Imperial: Calcutta-Bombay Route was of Imperial Prominence as straightway it connect Imperial Army from Britain to Northern India Revolt Belts. Lord dalhousie sanctioned Calcutta-Delhi as most important route while Bombay-Calcutta was sanctioned to be of Imperial Prominence!!
It never was a successsor & nor replaced The Imperial Mail, both been different!!
Purpose of The Imperial Mail & The Punjab Limited: To boost British Traffic by fastest possible connection between Ports & Mainland which was deprived by Regular Mail with multi stops & "Through Carriage" Links!!