Hi Bro,
I can explain as i have been travelling in this route since 1996 and have discovered some facts (something might not explain you as to why so??)
First and foremost is the speed limit of this route is so variable especially between VSKP and BZA where due to Godavari river and the adjacent 100 kms or so have...
more... been assigned less speed due to spongy soil area.The max speed allowed in this area is 80-84kmph.
Also between KUR-BAM due to GHAT Area and Chilka Lake by the side of the track for about 45-55 kms the train speed is not more than 75-80kms max.
Also the speed bewteen Howrah and KGP is lower bcoz ther are EMU trains plying which are given preferences atleast till 60 kms from Howrah.Then the train picks up speed till KGP.After KGP till BLS, the trains picks up speed only at some points and also it travels at an average of 75-80 while in patches 85-90.
BLS-KapilasRoad(13kms ahead of Cuttack) the train speed are pretty high around 84-90-95.
From Cuttack till Khurda Road the train speed is extremely slow bcoz of the Spongy soil of Mahanadi River and its larger tributaries and also Slack Times.
From BAM to Simachalam the train speed is extremely HIGH and Consistent to 85-90-95.
Simachalam -- VSKP -- DVD all trains moves at slower speed due to junction area and Slack times.
Trains picks up speed after KrisnaCanal Junction till Ongole and after Ongole for unknown reason it takes hell lot of time(1hr40 mins to travel 117 kms) till Nellore.Nellore to Gudur is pretty good at 85kmph.
From Gudur till Ashok Leyland Nagar the train speed is also pretty high around 85kmph.
So you can calculate your self to what extent trains can be speeded up and why in this route the trains are slower.Stiill 2841/2842 are the fastest in this route.