Modifying terrain element to fit props

Hello,

I am fairly new to Unity, and I have built my very first terrain, using a Perlin noise tutorial. While there are no problems with it, I want to modify it further to conform to a randomly generated railroad that I built earlier. Here's an example of my terrain and the railroad clipping through it:



Bridges are also involved in the generation, and need to be taken into account. The terrain is primarily quads, so I don't doubt that it can be done. In addition, the setup needs to be responsive to new track pieces and other components, such as the possibility for cities.