One way it can get approved is to have a licensed in Colorado Professional Engineer write a report with safety factor calculations and certified material reports as required by the applicable ASME codes and put his John Henery to it. That's the avenue that Alex Schneider and I took when we built our 18 inch gauge 2-6-2 we operated on the TNR now in Laurenberg NC. We built our own boiler and did get a Colorado boiler number issued by the state.
Roger Mitchell
Master Mechanic: Fort Collins Municipal Railway