I am not sure if this is a bug or just sloppy driving. On the Cajon Pass route, in the scenario "Held Up By The Highwayman" I never get too far without the trailers on the first 3 cars tipping over. I hit the first bend going down the hill and boom!! The trailers fall right off into an oncoming train. No matter how slow I take the corner I can't get past that point. Even if I get past that point, they end up falling off farther down the line on an even lesser bend. The speed limit in said bend is 35 mph and I have gone through at 10 mph and it still falls off. I realize that these are probably unsteady loads to begin with, but if they were that unsteady and unsafe, I doubt that they would be carried in that manner.
I've been having the same issue. I think it might have something to do with the physics of either the engine, the cars, or the couplers (maybe all three). I've installed this http://railworksamerica.com/index.p...m-sd75i-physics-mod-pack&catid=24:accessories and it seems to help a little. I would also apply the engine brakes before you begin moving to try and bunch the cars together, that seemed to work for me.
The patch did not work, I got an error when I tried to use it. The idea of releasing the train break and leaving the engine break on seemed to help. I ended up spilling the load on a point where the line goes from three tracks to two..., but it is much farther than I was getting before. I still think there is a bug with the physics model for those cars. I have the Marias pass route as well and with the same car had no problems on any of the bends on that route, which are far worse.