An Antarctic research expedition uses a heavy snow utility vehicle to tow a chain of cargo sleds across flat, icy terrain at a constant velocity, as shown in the diagram below.
The sleds are connected in series by magnetic hitch couplings, which release if the tension force pulling them exceeds a certain threshold.
As the team adds more cargo sleds to the rear of the train (to the left of Sled D), they notice that when the total number of sleds is increased, the magnetic coupling between Sled B and Sled C suddenly releases, splitting the train.
Explain, in terms of forces, why the coupling between Sled B and Sled C breaks when additional sleds are added to the rear.