Suggestion, add the company part # to your list, so if someone sees a truck type they want they'd know what to order

I know you said start with RB trucks, but since I model transition and older I'm also interested in FB trucks and passenger trucks and tender trucks and ...
Atlas makes a FB caboose truck, from their site:
N Friction Bearing Caboose Truck List Price: $3.60 per EACH
Item Code: 22061
The folks who make core kits, American Limited, make 3 passenger trucks, this is from their site:
8100 Outside swing hanger passenger car trucks, one car $5.95
8150 Commonwealth passenger car trucks, one car $5.95
8190 Pennsylvania 2D-P5 trucks, black, one car $5.95
ABOUT OUR TRUCKS
We have available the two most common types of trucks that were used with lightweight streamlined passenger cars. The outside swing hanger truck, 8100, was classified as a 41-CUDO with 8' 6" wheelbase or a 41-NDO with a 9' 0" wheelbase. These trucks, equipped with disc brakes, were used almost exclusively on cars manufactured after 1951. What we are calling the "Commonwealth" truck, 8150, is classified as a 8' 6" or 9' 0" wheelbase 41-N. There were many variations on this truck manufactured for cars between 1945 and 1951, including several with disc brakes (just clip off the brake shoes and omit the brake cylinder if your prototype has these).
Our trucks use Intermountain Railways 36" wheels and are designed to use Micro Trains 1023/1025 couplers with an adjustable mount. Mounting screws for trucks and couplers are included.
Also available, #8190, are the Pennsylvania 2D-P5 trucks, one of the PRR's standard trucks used from the 1920's into the 1970's on a variety of equipment: