In article , Chris Wilson writes
Chris,
The main reason for trans-shipping between trains is a change of gauge i.e. slate on the Festiniog. Otherwise laying an interchange siding is cheaper.
The main mineral chutes will be at collieries or offloading the coal at ports into ships of barges using the end door wagons. So side door wagons were for domestic coal, end and side door for tipping (and also for domestic, just not quite as strong) and bottom door for north east coal drops.
By the by, the door opening mechanisms at some docks would strip off vac. cylinders, causing BR to retain so many unfitted wagons