As far as diversion is concerned,then yes,it's not gonna be diverted.
But who told you that Spj-mfp-hjp section is main line section. it was providing loop for Delhi-up bound trains from north east India due high congestion on mainline(Bju-pnbe-mgs).
Now importance of SPJ-MFP-HJP section has been reduced after completion of BJU-HJP section.
Actually mainline is BJU-HJP-CPR not BJU-SPJ-MFP-HJP.
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And also all the premium trains are running through grand-chrod(Not covering major cities of Bihar like Patna,except Gaya). Even then trains are running through that section,even better section than HWH-Delhi mainline.