Minor historical interlude

Until relatively recently, the majority of freight trains in the UK largely consisted of wagons without brakes -- braking power was provided by the locomotive and a "brake van" (the latter usually coupled to the rear of the train).


But as locomotives got more powerful, freight trains got heavier, and they soon exceeded the safe braking capacity of the locomotive+brake van. So a new piece of kit was constructed that was, effectively, a brake wagon -- the low wagon seen next to the locomotive:


These brake wagons increased the braking capacity of the train, enabling heavier freight to be hauled:


They were soon superceded by the introduction of "fully fitted" freight trains, trains in which every wagon had its own set of brakes. Fully fitted freight trains could go faster because they could stop in a shorter distance.