I do not know if Purvottar Sampark Kranti ever ran via Bhagalpur. I had left Bhagalpur after 1995, only visit occasionally.
Bombay Janta Express (Bhagalpur to Bombay VT) was started in the 70s to serve Patna and Bhagalpur . It was was referred to as Bombay Janta. In those days Janta Express were trains with lots of stoppages. The priority was Super Fast, Express, Janta Express, Fast passenger and Passenger.