I have come to this thread very late, but I remember trains of timber wagons going past my parents house near Sleaford in the mid 1960s. The train originated at Boston Docks and from memory when new the wagons were painted a light green colour, and the train length varied, presumably due to the amount of timber on the latest ship to dock at the port.
If the destination was Colwick, it would make a lot of sense because the motive power was usually a class 25 and I always wondered where the train was going, as I never saw them anywhere else.