A while back, I bought a couple of these Selley (made by Bowser now) O scale hand car kits that would be "upscaled" to 1:35n2 for the Mogollon Railway. I thought since the kit was narrowed to 16.5MM gauge, it would make a neat little On30 or On3 bit of scenery. This was a very simple conversion and the kit wasn't very expensive (about 10 bucks). If you are just wanting an O scale version, you can build this nice little kit as-is or narrow gaugers can make a simple job and have something neat.
Here's the kit, Bowser #675-10424. It is a white metal (Zamack?) kit with just a few nicely cast parts.
First, I chemically blackened (Hobby Black, etc) all the parts. Then I drilled out the wheels since the axles go through them and then the frame is on the outside. In stock form, the wheels go on the outside of the frames. I didn't use a drill press here since it isn't critical to get the holes "true". There are little dimples where the holes can be drilled, I used a pin vise.
You can see how the frame, wheels, and axles line up. If you are doing On30 or On3, you are through with the modifications and can proceed with the kit assembly.
I added a scale foot to the pump towers for 1:35. No need for O scale.
I also added some 1/16" tubing pieces to the pump handles to make them longer and heftier for 1:35. Again, no mods for 1:48.
Here she is. I just made a larger deck to match the scale and a few doo dads. I have seen yellow hand cars so that's what color she got. The wood deck was stained and later will get some "stuff"-shovel, pick, little water barrel, and maybe a dude snoozing after a brisk ride.
I hope this will get you to try a kit which has been available for many years and with a little fooling around reward you with a neat little piece of detailing.
____________________ It doesn't matter if you win or lose, its' how you rig the game.