Carter Bros Passenger Car Truck, based on the 1895 SP shops drawing and prototype photos. Designed to accept 24" wheelsets mounted on 1.165" long axles: offered by Grandt Line, NWSL, and Foothill Model Works. To assemble, carefully remove the journal boxes and sprues from in between the truck frame...as well as the sprues which protect the brake beam guides/hangers during shipping and cleaning. After cleaning, the journal boxes should slide into the pedestals. Do not glue the keepers in place until the wheelsets are installed in the journals...the journals are sized correctly, but I like to check this with a drill bit. I recommend cutting the sprues away with a jewelers saw instead of a hobby knife.
These trucks also work in On30 so long as you have the 1.165" axle (NWSL and Foothill Model Works offer the appropriate On30 wheelsets on 1.165" axles).
A very similar truck, the Oahu Railway's Carter clone trucks, is also available.
Additional On3 passenger car trucks, and HOn3 versions, are in development.