I did not have a guards van for my railway so decided to model one based on the Glyn Valley Tramway guards van.
The body is all my own work although inspiration came from various sources on the Internet.
The chassis is a remix from the work done by Tebee https://www.thingiverse.com/thing:4499093 in order the produce a chassis of the correct length for my model.
The lamp at the rear is a remix of the one produced by Ironmink https://www.thingiverse.com/thing:5019977.

Bambu Lab

A1

Doesn't Matter

Yes

.08

not needed

It helps to print the roof vertically to eliminate layer lines but you will need a good brim, 5 or 6mm. You may also to wish to print the roof as a separate item so that the model doesn't ghost into the roof, which is why the chimney is a separate item, and also to minimise any stringing. You will need to flip the chimney 90 degrees in your slicer to print.
This model was printed with a .4mm nozzle and loses the handrails as a result. Better results can be obtained printing with a 0.2mm nozzle.

This is quite straight forward once the body is cleaned up and painted it is stuck to the chassis with superglue. I find that fitting the wheels to the chassis before gluing up helps as the chassis has a bit more flexibility at this stage. The roof is also glued in place after fixing the chimney with requires a 1.9mm dia hole to be drilled in whatever position you think best, in my case about 6mm from the one edge.
Wheels and couplings came from Ebay seller Woodworm2014

This was designed in Fusion 360.