Partially correct! Mahanadi introduced as Exp, then upgraded to SF (acc to IRFCA) in Oct 1988. Then once again it's category get changed to Exp during late 90s. Last time travelled in this train in march 2002 from DURG to BPL, then it was only a Exp train. I do remember we travelled in 2A, which was half empty. Then, I was just 10 yrs old (and hell yeah I know difference between SF/EXP that time also, born RF :P ;) ) along with my same age cousin sister we climbed every single empty berths, running like a crazy in whole bay area. Until we got scold by my mom. Departure time of train was around 10.30-11.00 in night. Train came with maroon coloured engine, may be WAG5 I'm not sure; at that time I only know two types of electric engine, one with straight face and another with pointed face....
more... That time I was not aware of WAMs, WAGs and WAPs.