My guess is the relative long length of the cars and the short length of the couplers are causing this. If you back it up slowly you might see that at a certain point the car nearest to the locomotive is forcing the car farthest from the locomotive to derail. There are two potential solutions I have used in the past. The first is that I use the longest couplers possible. The second is that I have had to not move longer cars on my layout. For instance I cannot run the long auto carriers in my 18inch radius curves.
Do I get a prize if I correctly assessed the problem? You have an “S” curve in the spur. My recommendation is to add a section of straight track as long as your longest car and/or locomotive between the curve to the left and the curve to the right. Unfortunately that will move the location of your structure.
I think you might be spot on however everything is limited to minimum radius of the curve. I had problems with 86 ft boxcar not able to negotiate the curves and I don’t know I call this issue my locomotive gets slowed by tight curve it’s like struggling but I fixed the problems with but longer curve
