...It works for "trackmodule.script". I guess some scripts aren't accessible to the loadScript addModifier 🤷‍♂️ Probably not anything in the game's res/scripts folder.

That's a *.lua file and not a *.script file, so the modifier won't pick it.
What do you want to do? If you want to override some function, take a look at the Town Tuning where I override some function for building eras. It might work for you as well.

Thanks @lollus , I'll check it out. Yes my confusion is partly from the modding wiki stating that "scripts" are .lua files (which is certainly true). But there is no detailed description of what is included/excluded with the different addModifier categories. So it seems like we always have to guess / experiment to find out 😖

Yes I'm overriding some functions in certain cases. So far I have been using a "wrapper" method, and it works okay. But recently I found another mod using a custom "package modifier", and I thought it might be a better way. But it seems it only partly works (it's code for a very early version of the game, so maybe out-of-date for current code base). So it gave me the idea to try the addModifiers instead because they seem to have similar intent..., but as you mentioned, I learned that it does not work on the .lua files.