Hi I'm thinking of building a 7 1/4" gauge steam locomotive. I want to run it on coal and use copper fire tubes. Possibly super heater tubes too. What diameter of steel tube for the boiler should I need and how thick should the steel be? Also if I was doing my loco in 5" gauge what size boiler. Normal steam locos have a fire brick arch inside the fire box. What is this for and is it needed on miniature locos?