Good photos to start.
I got the first 3 parts of the build your own Mallard kit from UK. Includes the Cab boiler front end and skirts. Have been wanting to build Spencer to go with my Thomas trains. I used the Lionel LionChief Albert Hall, (same as Hogwart's) chassis and tender and did a quick build to get it going for a show. It sits a bit high for now as the side rods will not clear the side skirts, will have to tinker to see if I can figure a way to get more scale sized drive rods to clear the skirting, and get the body to sit lower. The face is from a Lego Spencer, think it will be good enough for the kids to enjoy with the other Thomas characters by Lionel. I sure wish Lionel would consider doing a Mallard, and a proper Spencer for the kids trains. I think it would work well on the basic Hogwart's chassis with a few minor modifications.
A young Thomas fan running trains LionChief controls mounted to the board, with special covers I had made so they can not make the trains go too fast or snap the dials off. Keeps the trains only going forward less chance of a derail at the model train shows. I have now added a few sound units; chopped up some of the smaller wooden and die-cast Thomas and friends toys and put larger speakers from them into a speaker tube to get enough volume for the kids to hear the sounds at a noisy train show. With these mounted on the control board the kids can press and hear the talking engines where they are standing. (I cover the LionChief sound buttons, as the little kids get all 10 engines with bells going and drives you nuts and they cannot tell what sound button they are pushing.) Now have 10 Lionel LionChief engines for the kids to run at shows, and several other animated accessories I mount the controllers on boards on the fence for kids.
A really healthy train!!!!!!! Relative brought this along for a family dinner for me!! I love it.
A recent show set up I do with the 3 foam boards at the front with track mounted over the green carpet, have the trains and accessories that the kids control.