The Bachmann On30 caboose can be converted easily to 55n3. This is the basics.
Glue the weight onto the frame. This will stabilize the frame otherwise fitting the body will be difficult. Cut the lugs from the interior and glue them to the frame over the screw holes.. Bet you didn't know there was an interior.
Cut the areas shown a scale foot deeper. I used a 3/16" diameter burr in a motor tool to rough out the cut.
The lugs have two different sizes.
The lugs through the frame are cut to clear the trucks.
Cut the side even with the bolster.
The body will stay on from the friction fit with the frame.
Clamp a board to your bench and slide the caboose body over the board to slice off the excess side.
Slice off a piece of the removed side to make a threshold for the door. Then the colors match.
That is the basics of making a 55n3 caboose from an On30 Bachmann Caboose.
All that is left is cosmetics and mounting your choice of coupler.
Most nineteenth century narrow gauge cabooses didn't have ladders or roof walks.
(Click images to enlarge)