lvxferre, rok temu (edited rok temu)

If I understood the FFF correctly, that is not what they’re doing; they aren’t working with a half dozen straight sections, each fitting a single “square” (in this case a 2x2), and leaving curves up to the player. Instead they’re adding a new curve (the half-S) to be used with the other pieces.

For reference here’s how it works in Transport Tycoon / OpenTTD:
A cropped screenshot of OpenTTD, showing pieces of rail. Some isolated, some forming a branching railroad.

If implementing something similar in Factorio (note: the colours mean nothing, they’re just to distinguish the pieces of rail):
a Factorio screenshot poorly edited in Kolourpaint, to show the same rail concept as OpenTTD.

The later can be smoothed out to look a bit more like curves, but the basic units are all straight and diagonal rails.