While I've observed some O-gaugers are rivet and/or chuff counters, I've become an amateur boilermaker of sorts - this is not a beer and whiskey reference although the term does make me thirsty for a cold one. I think there's been some confusion on this subject so hopefully this example will make it clear.
Since I operate and collect mostly die-cast steam locomotives, I've noted over the years various models of steam locomotive boilers, specifically how some are rounded on the underside whereas others have a "U" shape - what one old time reviewer mockingly referred to as a "monolithic" design when he reviewed such models. Given the price of "scale" models, this has become a pet peeve, to a degree, since their are many factors in deciding to buy a model. From all the pictures of real engines, the rounded underside is the more accurate representation. The best way to demonstrate the difference in modeling a steam locomotive super-structure is by comparing two models of the same prototype.
First is a picture Lionel's model of the Reading & Northern Heavy Pacific. Note how the boiler casting drops down to an upside down "U" and where the arrow points to is the open end of the "U". Many of Lionel's scale steamers designed in the early 2000s utilize this design. On a real engine, you'd be able to see daylight under the boiler; in other words, this metal flange of the U doesn't exist on the real thing, and therefore, is an unwanted ugly piece of metal casting.
This is a model of MTH Premier's Reading & Northern. It has a rounded shape to the underside of the boiler, its painted black so doesn't show as nice as it appears in person; it's actually not a continuous curve all the way around the perimeter, but nevertheless looks much better than the U-shape design, and you can see daylight between the suspension and the bottom of the boiler.
I understand brass models mostly have fully rounded boilers, accurate suspensions, etc., yet there's no reason why die-cast models cannot at least have the appearance of prototypically accurate boilers, as the MTH model above demonstrates. Frankly, the U-shape design is unattractive, and is very noticeable, at least at eye-level, on Lionel's scale N&W J, PRR J1a, Berks, Mountains, Mikados, 4-8-4s, and Pacifics
I don't mean to pick on Lionel with the above comparison. More recent Lionel designs such as the Reading T1 of 2015 and the Milw. Road S-3 of 2010 have the rounded undersides, whereas MTH's Premier T1 has the "U" shape flange.
Given the never-ending discussions on prototypical accurateness, correct whistles, sounds, chuffs, etc., I feel that modeling a locomotive boiler correctly ranks up there with all the above. I recall that the "old time" reviewers in the magazines, use to note details such as boiler design. I for one, appreciated their attention to detail.
