I've tried it even after disabling every other mod and there's no extra GUI when I click on a locomotive.

Fantastic idea if you can make it work.

I checked the log and these are the only errors but I guess they are irrelevant:

63.892 Error CachedChangelog.cpp:37: Failed to parse changelog for mod Hermios Framework: invalid changelog file, error on line 4, category line does not end with colon.
64.468 Error CachedChangelog.cpp:37: Failed to parse changelog for mod Hermios Gui Framework: invalid changelog file, error on line 4, category line does not end with colon.

Hey @Chess
IT works for me, so i assume the issue is related to your game/configuration other...
Some stupid questions first(sorry, we never know): did you research the technology? Are you in editor mode?
If yes, could you send me your game saved? You can create an issue on GitHub with your problem and attach the file. Thanks

It works, but it requires a technology! (You should change the description)

Now a real bug:

When I delete a train station that I already had placed (before enabling the mod), it leaves behind a "trainstop-ghost-constant-combinator" entity and I can't place a new station in the same spot.

Placing new stations and removing them works fine.

Ok, will look for that later

ok, fixed the issue and added information about technology. Please note that your current game won't be impacted, as there is no retroaction for it.
But if you readd this mod later, during a game, the bug shall not occure anymore

That's ok, it is fixable with editor mode :D