If the helpers at the end provide more braking force than the head, the train will become stretched.

In fact, using helpers for braking was (and still is) common practice with heavy trains over the Lötschberg route in Switzerland (with 2.7% grades). The locomotives used are Class 465 units, which may produce the same braking force as tractive force at a given speed, when doing regenerative braking. Before the software change which allowed for that, it was generally about two thirds. Now, as the train is stretched, the helper engines can provide maximum braking force with a maximum of regenerated energy.

Note that such a energy regeneration is only possible with electric engines; diesels simply produce a lot of heat in brake resistors, which limits their braking capability.

FWIW, another "trick" which is possible with 3-phase AC drives (meaning motors without commutators), is to slowly stop the train in an uphill grade (for example because of a closed signal), and hold it without (or only very little air brake application). With that, it will remain stretched, and speeding up will be very smooth.