I started learning babylon.js more than a year ago and I just made my first simple tool with it: Easy 3d-tube creator available here.
It allows you to create combination curves simply by clicking at the tiles; you need three clicks to create the first segment. And you have to manually adjust the y-parameter either using the slider or keyboard presses (0-9 and “.” and “/” at the moment). You can’t easily create the sort of a curve you want but you can definitely create curves fast
And finally you can download a rather verbose json representation of the curve by clicking the save-button. You just need to write a constructor that can use that representation to create either tubes or roller-coasters inside your own project.
I became interested in curve-creation while coding my first semi-serious 3d-game CurveBusters and at some point I’m planning to do a more advanced version of it (using a physics engine rather than coding my own mechanics from scratch).