Matt, first, good looking layout.
Second, you really need to figure out the radius. Is the tighter turn a smooth radius? What I mean by that is the angle of the turn the same all the way around? In the picture it looks like it gets tighter in the center. If it is the same radius all the way around the turn, the essayist way to find out the radius is, is to measure across the U part of the turn. You need to measure from the center of the track.
Take the distances it takes to turn around and dived by half. If it takes 36 inches to turn 180 digress, it is a 18 inch radius. 30 inches it is a 15 inch radius and so forth.
Now for a rant on my favorite subject! You Don't Have To Have 22 Inch or Bigger Radius Turns To Run Steam Or Big Diesel Power! My layout, as case in point. I still have 3 18 inch radius turns that are hidden. Yet I can still run steam and 6 axle diesels.
BLI Steam. M 1 A , Heavy Mike and Class J. All so have a Hudson but it only has 6 drive wheels.
Bachmann 2 GS 4.
Mehano Santa Fe Type 2-10-2.
6 Axle Diesels
Erie Built, GE AC 6000's, RSD 15, E-6's, E-8, FP 45, SD 50's, SD 40-2, SD 7 and a few more than I am forgetting about.
Then the BIG Engines, Y6 b and the Big Boy.
2 Main Keys.
1 If the Manufacturer recommends a 22 inch turn, don't buy it if you have 18 inch turns. Reason I never bought a Bachmann Yellow Stone.
2 Have to have smooth and well laid track your!
With the age of your track, check for dips and high spots and if the rails ends are even!
One last thought, on the PA that wont make the turn. Is it by chances a Athearn Blue Box? Reason I asked is I had a Blue Box FP 45 that would not take a turn? Seems the front truck was hitting the ladder and caused it to derail. While the FP 45 is not a PA, I have a few PA's that I have not put in services yet and I see the ladders as being a possible problem.
Well I am off my soap box and hope I might have been a little help.