Yeah, I have been having issues with derailments as well, especially after the latest Fs25 update..
Interesting.. I am going to take a deeper look at how Zach did his and see if I can improve mine
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i also recently did a bit of a deeper dive and zca's amtrac doesnt like to stay on the tracks either, but his blue train often stays on just fine and is the one i used in my test. the problems on all trains only show on either uneven tracking, curves, slopes and switches from my tests as well (mostly).
also to add on, i had combined both your red truss bridge with zca's bridges because i thought it looked quite good and also gave the red truss a walkway on it and it worked fine a couple months ago, but recently (probably due to the fs update you mentioned) all trains but zca's blue one kept derailing on it for unknown reason (hence why i removed the trusses for now, allbeit i kinda want to put them back on as it looks too plain without it) and now when the trains pass over it it gets treated like a regular straight, so maybe the collision problem means that the train wheels keep clipping into the retracking rails in the middle of the tracks on the bridge, and probably on the same as the parts on the switches too.