I really know this kind of idea... laying out tracks or doing the balancing is the fun and easy part of it.
When starting a project like this, you should start implementing an engine prototype. Generating landscapes and rendering them in a performant way is really not as easy as it looks. I don't know how much unity is helping you at this point, but I have a lot of respect for the TPF terrain model.
For the scripting, have a look at ClearScript V8 (Javascript / Typescript). I've already deployed such a scripting environment for a commercial application - our customers are pretty satisfied with it. This will also come in handy in case you want to declare model files in JSON format, which is, I guess, the most common and best supported way nowadays. (LUA sucks and XML is old and bloated)
Thanks for suggestion on TypeScript, never used it but I have heard it a good improvement over Js. There is also some transpilator from OCaml to Js and ReasonML. I like ML langs (f#, ocaml etc) mainly for ADT and pattern matching for the expressiveness and simplicity of notion, and f#, like ocaml, have both compiler and interpreter, so the modder for complicated mods can easily get a balance between the performance and the quickness.
For the engine, I see the author of Mashinky has implemented his own engine, I don't know if I do in this, how many extra effort I need to put, I have never experience on coding a shader, a 3d engine, or sound engine, I don't know if it will help me a lot in later phase of developpement if I do an engine of my own, what do you think?