I'm definitely interested in this, huge fan of RRT2. I did in the past create a signal box sim (just a personal project not released in any way), simulating a British signalbox in steam days on Windows in C++ so have a little idea on how to code some but not all aspects of a Rail Sim as my sim had AI trains responding to user operating signals and points. I've recently been creating a very basic SDL2 (SDL is a open-source game API that is totally cross-platform, the open-source version of Theme Hospital called CorsixTH uses it) framework in C++ with the idea of porting my signalbox sim to that (the original was written in Windows GDI). The other thing I have is a book ("The Official Guide to Sid Meier's Railroad Tycoon" by Russell Sipe) which was a guide that came out with the original Railroad Tycoon which has a number of the equations (but not all) used in that sim in it.
So my suggestions on how I would approach this as an open-source project are:-
1) Any project would need a new unique name as the rights to the original are still active (if I've understood the conversation so far). Also unfortunately no rights or code means I don't think (correct me if I'm wrong) can easily create anything like a highres patch for the original.
2) It would be a reimagining of RRT type game and not be a direct copy (again to avoid copyright issues), so the above book could be inspiration but not following exactly.
3) Unless we have some great 3D artists sign up then we would have to consider starting with a topdown 2D view and using crude "programmer" art initially. ie a RRT look rather than RRT2. If designed right could then port to isometric later OR as someone suggested earlier see if we could get permission to use OpenTTD art as a starter.
4) For API I'd recommend something cross-platform so it can be played on many different OSes. SDL is good (its built on OpenGL but has its own 2D rendering engine built on top of that) except there is no UI API, so you either have to roll your own or find something on another open-source project that can be used freely. I also think Unity is also free for open source projects, I did a basic course in that so know how to do a simple game in that. Java could also be good as I gather there are some good APIs for that too but I not done anything in Java myself. Or could ask another open source project if we can use their framework (eg OpenTTD or CorsixTH), probably can anyway under open-source rules but always polite to ask!
5) As for coding then I think C++, C# (if using Unity), Java would be the main candidates with possibly scripting in Lua or Python.
6) Start small, we don't have any source code or rights to modify the original so iterative development using unit testing would be a sensible approach. So first create a 2D tiled map codebase, then add some predefined track, then add an engine to the track and get it to move along it, then stations, then work on solving the route finding etc ie step by step
7) Don't spend any money initially just have a free project on Github, so if the project hits the buffers lose nothing. I'd not recommend Kickstarter unless you have very experienced devs on board.
8) This would likely take years to complete! RTT and RTT2 are complex games so not easy to build quickly.