Some progress has been made, the floor, sides and ends are now in place.
Here it is with a Pepper 7 wagon for size comparison:
The back right side piece is too short, so there is going to be a repair panel added from a cut up tea light to disguise this.
I've given the rest a lot of thought. The original plan was to add a roof - the one from the Airfix OO Dockside Crane has a nice corrugated iron roof that would do the job well with a little modification, but it looks totally out of proportion given that it needs to be a scale 6' 6" (3 1/2 real inches) above the deck of the car so that railway staff don't hit their heads. I've put the roof away for a future project.
I'm going to add a vertical piece at the right hand end with a small workbench attached, suitably braced and supported, with some hand tools on it, and the usual tat under it which you would find under most benches. There is a 1:24 bench vise which (if I can find it for the right price) will go on the end of the bench. The other end will have a folded tarpaulin with a brick on top, odds and ends of timber, metal, rope and anything else I can think of to put on it to give the look of a working, used vehicle.
It also needs a step for the workers to climb up on to it with, this will probably be a bent piece of brass wire fitted to the solebar.
I've ordered link and pin couplers from our very own Teebee via Shapeways.
I'm also of the opinion that the overall colour of the wood is too uniform, as well as a very obvious black plastic chassis and shiny metal wheels, so some subtle weathering will be in order before too much detail goes on. This is a new thing for me, so the learning curve will be steep