On British railways, brake tenders were low, heavy wagons used with early main line diesel locomotives. One or two were coupled next to the locomotive to provide extra braking power when hauling unfitted or partially fitted freight trains (wikipedia).
This model represents those available up until the 1980s. Available in BR Green and later BR Blue. they were usually found on Common B4 bogeys (or gresley bogeys). the model makes use of the older farish coach bogeys (as found on the pullman coaches for example) which have a 4mm Hole and a 1.5mm pin to hold the bogey in place. the bogey has the coupling arm attached.