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Currently 3D printinga lot of Lego rail on my FDM printer. Although not perfect, it does snap onto original rail and to its 3D printed peers. Lego pieces do snap to the dimples as well, although too tight.

Main use case for me is to lay some track between rooms for kid's trains.

I have made compatible parts with my at home FDM printer.

I have too on my TAZ, but they either require a bunch of sanding to get the fit right or they are far too loose. I wonder if the tolerances on the resin printers are accurate enough.

extrusion tolerances can be tuned very finely on an FDM printer if you're using a geared extruder head and a well-tuned profile with a well known filament.

very high end FDM printers have been getting down to sub 100 micron dimensional accuracy for many years now.

if you're sanding parts you probably have some low-hanging-fruit to eat tuning wise.

SLA printer could do it, they even mention using it for prototyping on the wiki page

With SLA resin at $90 a liter, I don't know much much money you'd actually save.

Probably not as tough as Lego though