20" Curves and "modern" equipment

[randgust]

Despite my tolerance for sharper curves, I think you're pushing it on the distance between curves on that top "S".   By the time you add easements and superelevation, looks like trouble to me.   You've got to get enough room in there to get an 89-footer off the curve and flat before you start into the next one.   

And is that an actual spot you're trying to model or just throwing an S curve in there to be interesting?   You do have that around Seven Valleys for sure.

My ATSF layout has a specific spot I fell in love with decades ago, a reverse "S" curve on the main at Chalendar, AZ (between Williams and Maine), with the snow-capped San Francisco peaks in the background.   Slamming through an S curve with heavy superelevation on both main line tracks and no speed restriction, downgrade into Williams, through the Ponderosa pines.   The spot was the subject of the cover on "Canyonlands and Super Chiefs" book by McMillan, a Howard Fogg painting for UTLX,  and lots of calendar shots.  That was my excuse for throwing an "S" curve in for no particular good reason and learning to live with the consequences over the years.
https://cdn.shopify.com/s/files/1/0072/5972/products/Canyon_Lands_book_large.jpg?v=1332612194
I'll admit that pretty much the entire layout was designed to make that 'the spot', like PRR guys set up the room to do the Curve.

The telephoto shots on that curve are just fantastic to this day and the curve-to-curve distances are surprisingly tight if you look at the cars into and out of the superelevation area.    Glad I did it, but when you look at the track plan it does look like I made a mistake somewhere.

[OldEastRR]

I suggest making both curves of the "S" 30" radius or more. You can adjust the straight sections on each end to accommodate this. I'm not familiar with the NCR, but even if you're trying to duplicate a scene I seriously doubt the RR used there the equivalents of 20" radii, so you'd probably be making the model look more realistic.
Instead of a long tangent along the front of my layout I built the track to a very wide radius -- just guessing, it's an 8 foot or larger radius (I laid it by eye). The trains look really cool going through it. Most of my other curves are 20+" R. but I have a double curve with a 15" outer and 13" inner curve. That's only on part of that loop tho.

[ednadolski]

Light cars with body-mount couplers and short wheelbases (e.g. 89' flats) do tend to string-line if they're near the front of the train.

Yep, that's the pic, thanks!