This hugely depends on many factors. What quality should the rail be? Do you transport freight at 30km/h or pasenger rail at 200 km/h? Is there electification involved? How is the soil along the tracks? How many trains of what type do you need? Do you want electric train protection and signaling? How nice do you want the train stops to be? Does it cross property of private individuals that you need to aquire?
I just want to say that this is not really a question that has an answer since it depends on so many factors. You also have to think about regulations for rail traffic. Building a rail line in your backyard isn‘t neary as expensive as one you use commercially because of safety standards you have to comply with, which is a good thing.
Oh and even if you could specify a project, i still would have no clue since i‘m not a project manager nor civil engineer, just interested in rail and trains :D