(Disregards the following two paragraphs, my tired old eyes finally made out the 6 inch grid system you are using in the drawing, I thought it was a foot)
I’m seeing 18 radius curves in a lot of places, which will limit your ability to run passenger equipment, 6 axle diesel and big steam. That’s not condemnation, I have 18 and 22" radius curves on the mainline, 15" radius in the yard system, and down to 14" radius curves on one staging spur. I run mostly old time steam though. I get a little bit of valve gear noise on the 18 inch mainline curves from a Spectrum 4-6-0, but they run through them cleanly.
My kids run diesels, and have no problems with 4 axle motive power, but you can noticably see a GP-38 slow on the 15 inch radius spur system. We have one modern passenger train, with 72 foot lighted Lifelike coaches, and it traverses the mainline very well, those cars stay on the track as good as, or better than any other rolling stock I own, including some high quality Roundhouse shorties, and some others that I have put many hours into tuning, via weight, coupler upgrades, and truck upgrades. Not bad for underweight passenger cars with Talgo trucks and X2F couplers on tight curves and steep grades. I think I’ve had exactly one derailment with those cars, when I finger pushed a line of them through a turnout lined against the direction of travel.
Others here have recommended at least 2x the longest rolling stock you own of straightaway between elements of an S curve, but I do not have problems there on my layout, the 18 inch lefthanders transition immediately into right handers on the reversing loops and I don’t recall a derailment there, ever.
Where I do have problems is at all Atlas Snaptrack number four turnouts, (the frog is too deep and too long, lifting the opposite ends of the axles (diagonal across the trucks) off the rail, which allows the truck to pivit and the trailing axle to follow the wrong line exiting the turnout), Peco small radius curved turno
