Here in Oregon we have one of the strictist boiler codes in the nation. I should know, as I just spent five years (through an apprenticeship program, not just studying for the test) getting my Class V Boiler & Pressure Vessel license.
Anyway, I've been amused by the lack of inspections for "Hobby Miniature Boilers".
Here is the Building Codes Division partial exemption for model live steam locomotives:
Part II – Exemptions
918-225-0390
Hobby Miniature Steam Boilers
(1) Steam boilers of a miniature model locomotive, boat, tractor, or stationary engine are exempt from ORS 480.510 to 480.665 if:
(a) Constructed and maintained for hobby use, and not for commercial use;
(b) Has an inside diameter not to exceed 12 inches and having a grate area not to exceed 1-1/2 square feet;
(c) Has a working pressure not to exceed 100 psi;
(d) Equipped with an ASME-approved safety valve of adequate capacity, together with a water level indicator and a pressure gauge;
(e) The boiler is inspected and approved by the Division prior to initial operation; and
(f) The boiler is found by the Division to be substantially in accordance with the minimum safety standards for material, design, and fabrication in existence at the time the boiler was manufactured.
(2) This exemption continues as long as:
(a) There is no explosion;
(b) No alteration or repair is made to the boiler affecting its strength or safety.
(3) If alteration or repair is made, the boiler must be requalified for the exemption.
The bold about the explosions is mine, not the boiler code.
I do very little boiler work, my license is mainly for (large) commercial refrigeration installations. What boiler work I do is partially-exempt cooking boilers, and I wish Oregon required routine inspections on them. It is a lot easier for the state to force repairs and retirement of old boilers than it is for me, the contractor!