With all the new game engines I'm finding out the hard way that adding a world editor is NOT easy. I believe Trainz game engine was developed internally and likely the same can be said for MSTS. I'm not sure how Open Rails is doing it. Having GUI's (graphical user interfaces) isn't the hard part. It's actually getting a game engine to recognise those new creations and bringing it into the actual game.
I know a while back someone mentioned about how most simulators are missing what made MSTS and Trainz so powerful and long lasting is user content, and I agree. But from where I'm looking right now it's going to be hard to find a way to make it happen from a development end.
I'd be curious how other developers get around this, or how they go about doing it.
Thanks
Sean