हमसे सीखे कोई जीने का सलीका। सफर कैसा भी हो, मौज़ उडाये जाते हैं।। - Amir

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.