This is a printer-friendly modular train track set designed to snap together and apart easily with a toddler's strength, yet it holds together well enough to pick it up as a whole, meaning it won't fall apart as a child plays. It is compatible with the most common standard toy train sizes.

The pieces press together by placing the latch from one over the other and pressing down This downward press is easier for small children than high resistance when pushing together, like some commercial models. Some printers vary, so if on your printer it comes out too tight or too loose, the latch_tol variable is the tolerance parameter to adjust. Increase the tolerance to loosen the fit, and decrease the tolerance to tighten the fit.

At current filament prices it comes to about 30 cents for a regular sized curved or straight piece. A standard oval set would have 8 curved pieces and 2 straight pieces, bringing it to $3 of filament. There are stl files provided for half sized pieces as well, for a little more creativity.

To add in a little extra fun, I decided to print a set for my kid with glow-in-the-dark PLA filament.

Customizer note: Thingiverse happens to be running a 4 year old version of OpenSCAD in their customizer at the time of posting, and it does not support extruded arcs. The curved tracks are of course made of this, so only the straight pieces display right in the customizer.
If you want to customize the curved pieces, go ahead and download a copy of OpenSCAD and try it there.