Before the editor for the game was made, I came up with an idea that I had since abandoned; adding support for territory colors alongside the primary and secondary colors already found in the core game. Normally when you try to mix a primary-colored train with a secondary-colored train, you get a brown-colored "Garbage" train, which doesn't see much use outside of the plethora of user-made levels.
The original idea behind this concept: Instead of making a brown train when both a primary and a secondary were mixed, a territory color would be made instead. However not every primary +secondary combo would result in a tertiary color; if you were to try and mix red + green, yellow + blue, or yellow + purple, it would make brown instead of a territory color. Below is a diagram I made long ago that shows how the tertiary colors would function in-game:
As you can probably tell the idea didn't really take off, since I was the only one interested in it and it would've made the game even more of a brain-wreck since according to Matt, most players didn't get the color-mixing concept, even having trouble with creating the secondary colors form the primaries.