OK, here's the first draft - the track plan has been done in Templot, then a bit of colour added in Photoshop. The grid lines are every 3 inches. The whole diorama is just 4'x2'. The track enters from stage right under a brick-built overbridge (cliché yes but still the best form of scenic break, especially in such a small space). The 3 tracks would be fed from free-standing cassettes, positioned as required. The tracks fan out into 4 sidings - the rear siding passes into the works building itself. The siding in front of this passes between two buildings and off stage left. I might add an extra siding alongside this one, also running off stage left, so as to allow a loco to run around its train (making use of off-stage facilities at both ends in the process). The front two sidings would be quite overgrown - the tracks almost hidden in long grass. All around the perimeter of the yard there will be tall, mature trees, giving quite an enclosed, almost wooded feel (and providing an effective scenic break). There's possibly space for something at front right - maybe a small building and vehicle access?
The whole scene would be set up to give the impression of general neglect and disrepair while still being an active industrial facility (though in terminal decline). There would be various odds and ends of junk lying around in corners and between the tracks - rusting, rotting, crumbling away. Rolling stock would consist of open wagons (coal, possibly other raw materials - these would be loaded and unloaded off stage left, allowing full wagons to be drawn in and empties taken away later) and vans (goods outward, probably loaded inside the building itself). Motive power? Well, whatever is available really. Given that there need not be too many clues to location, this could be quite flexible, allowing me to indulge whatever takes my fancy - the theory being that the facility is served by the railway rather than being a privately operated affair.
Track and wheel standards will of course be P4, given that I have nailed my colours to the mast already! Mind you, I will have to try to recreate the appalling track conditions found in these sort of locations so that'll be interesting with the P4 wheels!