That old NTA standard...
I was just checking on this thread after a while, and a couple ideas occurred to me.
Firstly, the NTA (No Two Alike) standard of building that seemed to be used with steam locomotives, this may be what drives some of our enjoyment of these unique machines. The individual qualities of each different class of locomotive (or each locomotive), that were so specific to each railroad and class of locomotive, seem to be part of the attraction to these locomotives. To the crews, road and shop, who operated and maintained these wonderful machines this was probably a double edged sword. While certain modifications made sense on one railroad they became potential problems on another, however they made a specific class or locomotive a source of pride in that it worked efficiently in its specific job and was distinct in its application. To rail-fans and modelers this seems to be part of the beauty of these wonderful machines.
Secondly, while the steam locomotive is undeniably the symbol of the modern industrial age, they are essentially a hand built product.