Comment from Alan Payne:
These vehicles worked in what, in later years, would be called Block Trains, comprised almost entirely of these vehicles, with the odd extra coal wagon. From the mid 1950's the extra vehicles included the usual Railway Clearing House, wood bodied types, but later mainly the various 16 ton types and a few of the 21 ton types, including some older Loco Coal wagons.

These vehicles carried coal, mainly from the Nottinghamshire / Derbyshire coalfield, probably formed into train loads at Toton, for the Corby Steelworks, and always ran back north empty.

These vehicles made up the bulk of several trains a day, in each direction. They only ever carried coal in the years that I saw them, which is mainly from the Mid 1950's, being withdrawn from the late 1960's onwards.
The Northbound loaded Iron Ore wagons, where usually the BR 27 ton Tipplers, later some 26 ton VB fitted types, including some that belonged to Lancashire Steel Corporation, were used for this traffic. These Ore trains also included various types of Iron Ore hoppers, mainly of the 22T and 24T Roberts type. These returned South, to the quarries in the Northamptonshire area, empty.