The valve gear were the most challenging and rewarding parts of the process. Trying to fathom how the linkages moved depending on the positioning of the wheels and coupling rods to make it look feasible was hard. I'm not sure I've got them entirely sussed but I'm happy with the approximation. Then sculpting metal components in modelling putty in 3d. I had to build up layers between the wheels for the parts to sit on...
From top to bottom; 'Ya' class (Joy), 'T' class (Baker), 2 'Ovs' (Joy) and an A class (Walschaerts).