So, on my 6" radius track which I am going to use for a T-TRAK module (not the mainline ) I am planning on running B-Mann's Durango and Silverton starter set and my loco and three cars all have truck mounted couplers and they roll perfectly over it, but then I made an open air car for it as well and it will NOT roll on it at all (or it will, just one axle on each side derails, then it uncouples and then one side flips off of the rail - not ideal when you have a 400' scale drop into a river - possibly more of 100-200'.
It was based off of the Bachmann 40' Gondola (BN) and then I made the roof out of black cardstock. Here is a picture of a similar car: https://images.toptentravellist.com...n-air-car-on-Durango-Silverton-train-ride.jpg. I still have to cut a doorway on each side at some point.
I would flip one of the set cars over and then flip the open air car over and compare truck swings. It very well may be that the conversion car can only run on 9.75 inch radius due to the truck swing. You may have to modify the under frame to get more play.
By this point during my testing, on the 6" radius curves (KATO 20-174) if I modify the chassis to make it so the trucks can work, then I will still have my coupler problem and uncoupling on that 100-200' drop wouldn't be be the best. I was planning on modifying the chassis awhile back so that the trucks could rotate more but then I decided not to due to the fact that it isn't going to fix it 100% and I would still have to fix the coupler problem though it it would help even with the uncoupling problem or enough that I can ignore it, then I definitely would.
I don't have the set you are using, but the Bachmann gondola (I think) I have has more swing than the passenger cars I have that I think match up to what Bachmann put in their set, although Bachmann QC is such that your results may vary. I did notice that the gondola was very light which may be the problem (or at least a major contributor). Where in the train are you putting the gondola? Also, are the couplers Rapidos, or the over-sized Bachmann knuckle couplers? And, what wheel phlanges do the wheels have? I'll assume code 80 rail, since you mention Kato.......you can run anything wheel-wise on code 80, and I've seen times when simply going to "pizza cutter" wheels would solve some derailing issues. I'd try some higher flanges (if possible) and adding a little weight as a starting point.
The gondola's wheels have much less side to side movement compared to the coaches (the coaches can rotate nearly 180 degrees when the stairs are off). The couplers are the Rapidos and the gondola is almost the same weight as the coaches due to the added weight due to the roof. For the wheel flanges, I have whatever B-Mann put on. I haven't tried to have any "pizza cutter" wheels yet, although I might get some next time I am at a train shop that has them. I will probably get a pack of N scale people and through in 30 or so just to have it more detailed for my seine there is always at least 30 people in that car!. I have the gondola placed as the second car (0-6-0 (being converted to 2-6-2), tender, columbine, this trouble-prone gondola, and two coaches (I might get another at some point to have three at the rear). At some point in the future (somewhere between a month or 2 years from now or never) I will be converting everything on this train to Nn3, so I am just trying to find the cheapest solution (I already have truck mounted couplers and trucks) to get me through until I get it converted to Nn3.
