My late uncle also had a great idea which utilised the Airfix loco kits. He used to build them in two halves, then, eg with the City of Truro, paint one half in GWR livery and the other half in BR livery. He would then get a couple of pieces of hardboard paint it in a neutral colour then mount each half and put a wooden frame round them, a sort of 3D picture, and hang them on his study wall. They looked quite effective.
I particularly like his treatment of Evening Star with a matching grimey 9F.
That was the beauty of these kits, because they were so cheap.
In general getting the wheels, coupling rods and pistons to move, however, was another skill, and required a lot of graphite and a great deal of patience.