If you want to get something of yours runnning on the cub layout, I think it makes more sense to build a box-stock 60 foot Intermountain flat car and spend your creative effort on the custom load.
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The Intermountain car will run well, and it is very nicely detailed. You will learn a lot from building it, and it fits in the club era.
Do you mean the 60 foot flat cars similar to the Trailer Train flats Intermountain made as kits as well as ready to run? I’m not sure that I would recommend them. I have several of the kits, and every one of them has a sag in the middle of the frame. And I was looking at some ready builts at a train show last weekend and every one of them had the same sag. And I don’t know how to get rid of the sag.
I have no idea why but I love oversized cars, guess the KCS super gondolas kitbash stems from that, and the club has regular flats but a heavy duty flat would be something different.
I’d probably paint it in NWP SWP colors, I haven’t done the KCS gon yet a guy at the club wants to tinker with it to see if he can get it to run.
I did not know about that problem with this kit. I have never assembled one myself. I have assembled many other Intermountain kits, and they have all been great.