Here is the standard used by the antique boat people. It strikes me as a reasonable standard that historic railroading could adopt.
“PRESERVED VS RESTORED
The Antique and Classic Boat Society (ACBS) established definitions of “preserved” and “restored” boats which the society uses when judging an antique or classic boat:
ACBS defines preserved boats as those containing at least 60% of their original deck and topsides material and is constructed using the same methods and materials as the original. Bottom replacement is expected in order for the boat to be serviceable but the method of replacement must duplicate the original….
For a boat to be considered restored, its owner must…provide photographic evidence of the existence of the original identifiable boat and of the various stages of the restoration demonstrating that the original boat was always together as a single entity…At no point should two boats exist – i.e. a pattern boat and the new boat even if the pattern boat is subsequently destroyed. Building a new boat using some wood from an old one will not qualify as a restoration. [https://acbs.org/acbs-boat-classifications-judging-classes/]
But ACBS concludes with the following, definitive, statement:
The amount of original wood in a restored boat is not determinative. For example, the USS Constitution has essentially none of its original wood but we believe no one would consider it a replica. It is Old Ironsides.”
“The U.S. Navy’s philosophy mirrors that of the Antique Classic Boat Society, the ship has always been Constitution and will always remain Constitution, regardless of how much or little 1790s materials exists in the ship’s structure."
“The jibboom that was removed from Constitution prior to the ship entering Dry Dock 1 in May, 2015, was manufactured in May, 2003, from laminated Douglas fir. It was refurbished in the 2007-2010 (floating) restoration and re-installed in 2010.”
Regarding #191's boiler, regardless of the material, it has an
unreinforced lap seam barrel, and as such is subject to rather draconian rules to operate under the FRA. (The ESC all agreed that lap seam boilers shouldn't be used, but they didn't want to put an absolute ban on them that would stop some really historic piece from operating for special events.) So the rules require that the lap seams be inspected both inside and out every year, i.e., the jacket, insulation, and flues all needed to be removed every year, something that would be economically untenable for most operators.
Considering the discussions posted above about #191's condition, it sounds like a no-brainer that she should be undisturbed and as a pattern for a reproduction.
At my former employer, we built locomotive boilers (and by the way, a straight barrel, steam dome ahead of the firebox boiler would be easy-peasy to build compared to the wagon top, crown bar boilers we had been building. We have also designed, had cast, and machined new cylinder blocks, completely new driving wheel sets, new frames, etc., just not all for a single locomotive. So the bottom line is that building a faithful reproduction #191 is something easily accomplished, all that is needed is money, and honestly, a reproduction may cost less than a restoration, without the need to carefully disassemble and decide how to handle each piece of historic fabric.
