I'm not sure I understand your argument. Do you mean turning a single-track line into two parallel single-track lines, with shared passing loops? Or do you mean turning a single-track line into one double-track line, with all train types using both tracks? In either case, I note that the very use of passing loops is a limiting factor (making freight much slower), and you need the more the bigger the difference between top speeds.

_{*Lunatic*, n.
One whose delusions are out of fashion.}