So I'd actually been sitting on a beta update for a few months now, wherein I added an optional slider to adjust the alpha channel of the trains' colors. It doesn't quite achieve the desired increase in vibrance though, because while a lower alpha value is more saturated, it's also darker; on the flipside, higher alpha does lighten up darker colors, but generally they end up being washed out. Because different colors behave differently, it's really very hard to settle on an alpha value range that looks good for everything. It seems it's best to just stick to the default 0.5 level, so I scrapped the alpha adjustment option.
Now the trick to the Vibrant Trains mod is it simply uses a different set of color mask images for the locomotives with lower transparency than the vanilla masks. I agree, they make the trains look much better, especially for colors in the yellow to red range which are more muted than they intuitively should be under default settings. All that said, Vibrant Trains works seamlessly with this mod, so I'd just recommend using them together to achieve this result.
