I've been playing with some slightly bigger versions of the tippers in question.
They are originally Bachmann G scale tippers - the big brother of the On30 tippers.
They (5 off) have been re-gauged to Gauge 3, and am in the process of turning the timber detail into a representation of the side tippers at Cottesmore.
Although not quite so rusty.
A bit more detail and then black to start with, and a bit (lot) of weathering.
Rgds
Richard