Karnataka express - 1977 ರಿಂದ ಸಿಲಿಕಾನ್ ಕಣಿವೆಯ ಭೂಮಿಯನ್ನು ತಾಂತ್ರಿಕ ಮನಸ್ಸುಗಳೊಂದಿಗೆ ಪ್ರಜಾಸತ್ತಾತ್ಮಕ ಪ್ರಧಾನ ಕಛೇರಿಯೊಂದಿಗೆ ದೇಶಭಕ್ತಿಯ ಮನಸ್ಸುಗಳೊಂದಿಗೆ ಸಂಪರ್ಕಿಸುತ್ತಿದೆ - Sakthivel Srinivasan

Coupling strain is not just on gradients. It can happen for very long rakes on level ground as well. You can slightly modify the rope example given by another blogger to understand this.
Imagine a person trying to drag a body with a rope tied to it. Now, the person has to be strong enough to move the body. And the rope also should not break when the person is pulling it. If the body is heavy, then more force has to be applied on the rope and the chances of the rope breaking increase. It can happen on plain ground also not necessarily on a slope. Just that on a slope, the chances of breaking are higher...
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for lesser weight than on level ground. Now if another person is pushing the block from the other side, then it becomes easy to pull the body and since both people are sharing the force applied, the force on the rope will also be less and heavier blocks can be moved with the same rope. The rope is a good analogy for a coupler and the helper is analogous to a banker or a middle loco.

The first thing is what was applicable in the starting thread!!
2nd thing prevails & very mch exist (& has been repeatedly highlighted by u..whch i only counter as nt mch applicable over this thread)!!
Yes..If Rake length too high (like 342/650 as in Iron Ore example in RDSO PDF)..Traction & Coupling Force bth will prevail & multiple loco need to b installed (either as banker thru push...r Mid Loco installed for the same to boost communication via Locotrol--cz it may hav sme limitation for distance & may nt handle 2-3 Km stretch...r...
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mid locos with LP while the last will throttle first with 2 notch & thn the second last n sequence till front loco:::applicable as per mechanism available & ease of split of Rake)!!
But currently such requirements limited over high gradients as in specific location..as such high tonnage haul doesnt happen in nill/low gradients @ India!!