As far as I know, the 5900's had Andrews trucks and the frameless tank cars had Bettendorfs. The 5900's Andrews trucks were the same as those under the long reefers. The reason that the 5900's were scrapped was that they wanted to use their trucks and hardware on new pipe gons and flats being made from retired standard gauge box and stock cars. Thus, you might be able to find a set of the Andrews trucks from a "retired" 6600 flat or from one of the former pipe- now rider gons on the Silverton (weren't these given passenger trucks?). Also, maybe one or more of the narrow frame tank cars is presently riding on a set of Andrews trucks that may be going to 5995 in the truck swap? ...Or maybe the Bettendorf trucks will work under a 5900 car and they are planning to use the Bettendorfs until they find a set of the correct Andrews?-- just my thoughts.