I would like to have trails on all of my trains, but I am too lazy to change the color of each of them. Would it be possible to add trails to default colored trains?

Yes I can make that happen. The reason it doesn’t is because technically a train with the default color actually has no color, and some modded locomotives have default colors other than red so I didn’t want to assume red for all of them.
But I think I can add a setting where you can choose a color for default locomotives (or enable rainbow trails for all trains)

I would like to have trails on all of my trains, but I am too lazy to change the color of each of them. Would it be possible to add trails to default colored trains?

This has now been implemented for version 0.0.6, thank you for the suggestion!
A setting has been added, "Default Train Color", to choose what color the trails will be for trains without a custom color already set. You can also chose a rainbow instead of a single color. It is turned off by default.