When using the Unity Tool to drag over an area, straight rails cost just about 0.1 unity per length (single cell). However, when selecting the build ghost for a rail segment, the unity cost to quick-build it using the button scales differently. For segments up to 25 units long, the cost is greater (e.g. 1.1 unity for length 8, 1.9 unity for length 16). For segments over 25 units long, the cost is less (3.0 unity for length 32, 4.5 unity for length 64).
This means, when unity-building long lengths of rail, the player is encouraged to create gaps in the rail, in order to split the rail into segments that they have enough Rail Parts to quick-build. (The sensible method of using the unity tool to drag a box around a chunk that can be built at once costs much more unity.) This sort of annoying busy-work doesn't seem like the kind of thing that should be encouraged with unity savings.
