In a bit of a rush so I haven't read any of the other replies so I apologize if this has been mentioned. I have successfully built 3 buildings on top of RR. I even started a thread about it a long time ago.
A few tricks that I learned are start with steps on top then work your way down, set up a bar vendor up there, and assign settlers to beds located up there.
Steps: There are those porch steps that are only two steps tall. Start with those. Put them somewhere where they will turn green then snap a 1/4 section of floor to the front of it so it hangs over the roof. Then use the wide stair that will go in the ground and snap those until you reach the ground.
Bar: Build one and assign a settler to work at it. Set up some chairs and tables. Settlers are attracted to bars after work hours (8pm iirc?). You'll soon have more settlers up there than you want.
Beds: Assign settlers to beds you put up there. Happiness will go up if you build a nice building and section off rooms with beds and some decorations.
TV's and Jukebox's are glitched and actually reduce happiness so avoid those as decorations.
This is how I have done it several times and it worked very well for me.
hth and sorry again if any of this has been mentioned.