Some time ago I built a narrow gauge (SM32) Wickham railcar from an IP Engineering kit. I'm really pleased with the model, but it needs a trailer to carry some tools. Searching the internet for inspiration I came across a picture of a 2-wheeled trailer for a Wickham trolley on the Bala Lake Railway (scroll down for the picture). This seemed such a crazy idea that I just had to build one!

As with my other narrow gauge wagons, this model uses Rugga axleboxes and 16mm disk wheels from Binnie Engineering - they are a perfect match to the original trailer.

Construction is straightforward:
Print Trailer Frame, Trailer Subframe (this will need support) & A-frame, then glue the subframe and A-frame to the bottom of the main frame. The very shallow cutouts in the bottom of the main frame should be visible enough to guide the position of the other components. If you want to fit a stand, print the Stand Bracket part, tap the hole with an M3 thread and glue it to the side of the A-frame. Paint the frame before adding the floor and sides. I used red oxide primer like the original.
Print Trailer floor and 2 each of Trailer side and Trailer end. If possible, adjust the infill orientation (or the orientation of the parts) in your slicer software so the top and bottom layers have infill running along the length of the planks. You may need to adjust the layer height to get the grain in the right direction on both the top and bottom layers. With a relatively coarse infill setting for top and bottom, a single perimeter, and using wood PLA filament if possible, you can get a very convincing wood effect. Fit the floor first, then the ends and finally the sides.
Then add the axleboxes and wheels. For the stand, file or grind down the head of an M3 machine screw to give a thin disk, screw this into the stand bracket and finally glue the handle in place.
The model is very light, so it's worth adding some ballast under the floor to help keep it on the rails. I used a small piece of scrap lead fully encapsulated in epoxy, but a few penny washers could be glued to the underside of the floor instead.

Of course a tool trailer needs some tools, so I added some new models and used others found on Thingiverse.
Platelayer's sledgehammer - print the head, plus two copies of the handle (ideally in wood PLA). Paint the head, glue the handle halves together then glue the completed handle into the head.
Pickaxe - print the head with support; construction is the same as the sledgehammer.
Screw Jack (working) - print the head, body and bar. Cut the head off an M3 machine screw and glue the end of the thread into the jack head, leaving the cross-holes open for the bar to fit into. Glue an M3 nut into the top of the body, then screw the parts together.
There are also some wood blocks (use the same trick for the grain here), a pry bar and a fishplate spanner.

The other tools are:
  my open / closed toolbox remixed from the original model by Enely (the spanners and screwdrivers are from this original model)
  the Scale Accessory - Chainsaw and Scale Accessory - Jerrycan, both by Burnie222

You will probably need to adjust the scale of some of these other tools. Many thanks to the authors for their excellent models!

Finally, the wheels provided with the Wickham Railcar kit are plain steel disk wheels which don't look quite right. I've included wheel trims which give the appearance of the pressed steel wheels often used on these vehicles. Just print 4 of them and slip on to the ends of the axles.

Disclaimer - I have no connection with either IP Engineering or Binnie Engineering, other than as a satisfied customer.