Fortunately things are easier now since the ASME adopted a new locomotive boiler construction code a couple of years ago. Before that it was challenging to try to satisfy two masters in the ASME and the FRA who didn't always speak the same language or see eye to eye on boiler matters.

Any new boiler project should begin with a healthy design and engineering budget to hammer out all the details before cutting, forming and welding begins. Designing a boiler as you go or simply copying an old boiler and assuming it will pass muster is a recipe for disappointment and bankruptcy. Any way you look at it a new boiler is going to cost significant $$$.