Restriction by BSL division of CR.
Similar restriction is being imposed on WAP4 locos as well.
431 05-Jun-18 12715 Dir 15 CR BSL 22813 BRC WAP4 Conventional Miscellaneous (NED-ASRT/Load-24 Coaches), Detained between SXA-DGN station for continuous wheel slipping on gradient 1:110 with 2.17 degree curvature due to heavy rain and visibility poor, LP applied small stone chips and manual sanding then cleared section. TOH-18.04.17, IC-19.04.18, Major/Sec-BSL-KNW, Minor/Sec-Sagphata-Dongargaon (SXA-DGN), Time-20.31-21.00 hrs. 1 12542/19, This is a graded section having gradient 1:110 with 2.17º curve. Due to rain, wheel slipping occured continuously while negotiating...
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this section, speed gradually dropped and train stopped at km 540/28. After sanding manually and applying small stone chips on rail train started. Same loco worked further with the train. WAP-4 and 5 not suitable to haul 24 coaches train on CR. Hence advised to all concerned Rlys to modify the link. Traffic/wrongly logged

Thanks for the update.
This looks rather hypocritical though. For the conditions in this situation, any loco could have had wheel slip. It is puzzling as to how they concluded that WAP4/5 are not suitable on these sections.
Check out the recent WAP7 wheel slip cases-
"863 03-Jun-18 22846 Ind 35 CR BSL...
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30356 TKD WAP7 3-Phase IOH, TKD, Aug 4 2017 / IB, TKD, Mar 10 2018 Miscellaneous (T.No.12628 NDLS-SBC, Load- 24 Coaches), took extra time between PHQ-KJ for continuous wheel slip at gradient 1:200 due to rain. After observing caustion SR 40 & 30 kmph speed not increasing and speed dropped upto 4 kmph. Manual sanding done and section cleared to KJ and precedence of one train. No loco trouble. IOH-04.08.17, IB-10.03.18, Major/sec- BSL-CSN, Minor/sec- PHQ-KJ (Pardhade-Kajgaon), Time- 17.55-19.26(91-20=71" extra in section) & 19.26-20.03=37" extra at KJ. 1 12166/42, After observing CDO of 45, 40 and 30 kmph at different locations speed did not increase and further dropped up to 4 kmph due to rain and wheel slipping. Train moved up to KJ loop with manual sanding. Train further detained at KJ for precedence to M/Exp trains. No loco defect . Traffic/wrongly logged"
1 01-Jun-18 16863 Dir 30 CR NGP 30546 LGD WAP7 3-Phase DOC, LDH, Aug 31 2017 Miscellaneous (BGKT-MQ, Load- 19 Coaches), took excess time between MALK-BYS for continuous wheel slip at gradient 1:170 due to heavy rain and speed dropped upto 3-4 kmph. Manual sanding done by ALP and section cleared. No loco trouble. DOC-31.08.17, IT2-22.04.18, Major/sec- ET-AMF, Minor/sec- MALK-BYS (Malkapur road-Barsali), Time- 14.30-15.04(34-5=29" extra). 0 Train lost time in section due to rain and wheel slipping There was no loco defect. Traffic/wrongly logged

Seeing as to how the P7 too is facing similar issues in rainy conditions (and in those two cases posted above, on milder gradients and in one case only a 19 coach load), it is puzzling as to how CR concluded that it is a problem with the locos. At least these two incidents definitely do not show any smooth operations. Particularly, in the second incident, the loco was an IGBT loco, which is supposed to have even better wheel slip control.
Also SCR operates 24 coach trains on the SC-KZJ section (which has a lot of frequent gradients) with WAP4 locos and as far as I know, they have never issued any such restriction. And in fact,...
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in the same section, WAP7's have stalled multiple time because of wheel slip with the 22 coach LHB SBC Raj and the 24 coach ICF Telangana (AP) express, in rainy conditions. But both P4 and P7 continue to operate freely on this section.
It will be interesting to know how what forced BSL division to make this decision.