You have better time to remake your surface in a solid! ;)

As you put the little solid inside the big no matter if the internal curve of the little solid is not absolutly like your first one ;)
Now the boolean Union will works like a charm!



And the fillet a game's boy ;)


Of course I take only one rail from your model, just retake any number you want ;) 2 is the maximum for the rails except the possible third scaling rail ;)
Else you must make your Sweep by parts and merge all at the end!