Train Groups seems to have attempted to add this feature. but i cannot get the newest version of that mod to activate in Factorio.

maybe it would be possible to add/remove a station to all trains with just a lua script?

kind of. this mod would be great if i had it from the beginning. if i were to use it now, i would need to add each existing train to a group one at a time. this is not an option, i have too many trains for that. i need something that will work with an existing network of vanilla trains.

that mod ...

I wish there were a way to give a command or edit the schedule of, several trains at once. perhaps some of the early concepts would be done via adding buttons to the edit station menu. like a button that does "add this stop to ALL trains". then maybe even add a "add this stop with X condition to ALL ...