I cloned a tender wagon for practicing Fusion 360.
Printer: Print Rite DIY
Infill: 20 %
Printed in PLA.
Wheels are better if printed in resolution 0.1 mm.
Based on axles are 2 STL files. You can choose between M3 Bolts axles or 3 mm diameter rod.
I personally prefer the one with bolts, no cutting easier to find, easier assembly.
Bolt should be 12-15mm long
Rods 30 mm long
For magnet connection between wagons I suggest
checking solution from this design: