When rendering the blends of the Mark 2A carriages, the roofs end up with a graded tone, being light at one end and dark at the other. Does anyone know what causes this?
Yes - this is caused by the mesh's faces having a setting that was used when Blender rendered things a little differently than it does now. You need to select the carriage roof, press Tab to enter edit mode, select all (ctrl +A) and then from the right click menu, select faces > shade flat. Thus should fix the problem.