GTL-DMM has a lot of 1 in 100 gradients which made it difficult for freight trains to clear the section. That's why electrification of the route was sanctioned.
In general, the WDM3A/D locos have inferior performance compared to the WDGP/G4 locos in all parameters. You can be pretty sure that if a WDP/G4 is unable to do a particular task, then a single WDM3A/D loco will not be able to do it at all.
The Creep control in EMDs is mainly relevant when facing adverse track conditions and when starting trains....
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In normal conditions, trains clear gradients simply because of the speed at which they are traveling.
A single ALCO cannot maintain any respectable schedule with a 24 coach ICF rake. The general observations is that trains with more than 21 coaches preferably get twin ALCOs to maintain their schedule.
The performance comparison is generally in the trend of 1 ALCO 1 EMD twin ALCOs twin EMDs.

It's not that a single ALCO cannot haul that load. It's about maintaining a schedule and also the amount of wear and tear the loco is going through.
With a single 3000+ HP ALCO hauling a 1000+ ton ICF self-generating rake, there is practically a very small amount of acceleration reserve left. If the train goes slightly off schedule, it will be extremely difficult to recover. The loco will also go through a lot of physical stress during this and will be more prone to failures because of the excessive performance demand from it.
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is why SCR typically uses ALCO twins for most of it's longer rakes, as opposed to a single loco. Trains like Venkatadri express used to run with a single WDP4D and then switched to twin ALCOs as a single loco was being put through a lot of stress to meet that schedule.

Sir, are you sure about this statement ? During its diesel days, BSK used to regularly clock ~75 kmph avg with the good old ALCOs (usually SPJ WDM-3D) between GKP-LKO, despite plenty of speed restrictions being present + super crowded 24 ICF coaches. Although it stressed the poor ALCO a lot :-P
I've personally experienced BSK covering GKP-LKO section in flat 3hr 45 mins with a single WDM-3D.
Another personal experience : /blog/post/3630674
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Although ALCOs are technically inferior, but these old workhorses are still worthy enough to maintain decent schedules.