I have a real beginner's question. As a long time operator of American Flyer, I have no experience with Williams trains. I do however have older Williams  F3 s and a Trainmaster that were purchased as shelf display models. I am rebuilding my layout and finally adding some 3 rail to it. What transformer would I need to run these? AC? DC? If I need to add DC, could I wire a switch so I could switch between either the AC or DC transformer as needed? Thanks for any information.