IMHO NRT should change the game the less possible, we should consider what you can do with the base game now and what you can do with NRT now.
I'll reply inline
I see two options here:
1. Stay the course with an improved error message. totally agree
2. (WARNING - Includes philosophical thoughts)
- How do competing players currently deal with tram track&catenary removal? how do current base game players deal with "player 1 removes a road used by player 2 too"?
- Why should trolley catenary removal be different? it isn't, trolley catenary is a road type and has nothing to do with tram, they are completely indipendent, this is why we decided at some point to draw both catenaries (leaving the burden to the grf authors to make them look nice) because players couldn't tell if there was catenary for tram or for road in one tile (ROAD+ELRL or ELRD+RAIL)
- If a competing player accesses either or both of my catenary systems, does that player contribute anything to my build/maintenance costs? nope, NRT doesn't change the way these things work, only extend it where they didn't exist before
- The competing player accesses my catenary system at his/her own risk. No? yes, same as first point
- My removal/relocation of my infrastructure is done for the efficiency of my network, not that of the competing player. No? agreed
- I should be able to build restricted infrastructure (e.g. andythenorth/HAUL). No? uhm, what do you mean? You MUST be able to build restricted infrastructure
- A solution? Road ownership is the key. It is company or town owned. The owner inherently sets and grants traveling privileges but not modification privileges, with the exception being trolley and/or tram service added to a town owned road. That town then still owns the road, but the trolley and/or tram service belongs to the building company who assumes exclusive trolley/tram modification privileges. A player can't add trolley or tram infrastructure to another player's owned road.
The last one is the real question: a solution?
We should need an owner stored somewhere which tells who made a modification so the only one which can change it is who did it.
We should allow free conversion of town roads, with all the risks and abuses it might bring on?
We should limit it in some ways? (currently is like this, maybe too much restrictive)
What could happen when towns will be able to use their own preferred roadtype instead of always base road? (and I already have a patch for this)
Should we change also how one way roads is handled? (currently you must own a road to make it oneway, so the ability to convert town roads without rebuilding them is even less restrictive than oneway roads)