My $ .02- (often through inflation- being $2): modeling trains is more "suggesting" things than duplicating things in scale. Everything appears better on the biggest turns you can fit in the layout. The extreme would be to go to flex track- and make "scale turns" - the Horseshoe Curve in SCALE would be great-looking. That would be 27 feet across in your spare room (1300 feet in 1:48). BTW the Horseshoe Curve is considered as very TIGHT in real railroading terms. See: https://ogrforum.ogaugerr.com/...-curvature-or-radius
So- go with O320 curves to model a "tight" curve. (That's a joke son!!)
Most model O locomotives REQUIRE (to avoid derailing), either a 054 or 072. BUT, I have compromised:
There are a LOT of locomotives that are OK even at 036. This especially if you are willing to accept a "traditional-sized" smaller than full O SCALE engine.
On 036, many times 048 even 060 turns mean you cannot run modern-era or even 1950's SCALE six-axle diesels, or WWII prototype articulateds, since they might derail. Go with the manufacturers' recommendations on minimum radius turns. Also consider the clearances required for tunnel entries and the like. I have a 773 Century Club Hudson- 4-6-4 and I had to trim the back corners of the cab for this reason.
But, you can, (in Lionel) run postwar traditional-sized locomotives, smaller SCALE steamers like a Pacific, or a first-generation 4-axle Legacy diesel - F3's, F7's GP7's, GP9's RS-3 etc. In passenger cars- SCALE are 80' or 20- 21" cars that might RUN on smaller-radius turns- but the overhangs make them look silly. I run 16" cars and nobody even notices.
In freight cars- these can be tricky- 50-foot scale cars or "traditional" cars- can look good even running together. The length I have found looks best is no longer that about 12-1/2" actual length. Then run longer cars behind a larger engine or as a group instead of mixing them into a bunch of "traditional"-sized (usually 10-11") cars.
- my 048 section is hidden under a tunnel and mountain- the "in your face" sections of the layout are 072 for appearance.
- I set my era as 1950- so shorter diesels look right.
- I bought 16' passenger cars, and;
- I gave up on having articulated engines.
The one thing you cannot put up with (I think) is difficult or impossible maintenance or reach, taking into account the persons doing that. So- for you (using a duck-under as you get older) and our family (now), a duck-under requires good knees, and clear, clean wiring so you don't snag and disconnect anything while you crawl. And a lift bridge allowing entry to the center of a layout might be a difficult engineering project, expensive, or a pain- all just to reach a car that slipped off the tracks causing a short.
I have not regretted my choices, EXCEPT- I should have had 072 easement curves leading to 060. And I have been VERY happy that I purchased a topside creeper.
