@Clarity - yes, they can.
The basic idea is that you spend about 3 or 4 years as a Fast Streamer, with a view to being promoted onto a position at Grade 7. Grade 7 is generally the grade just below the Senior Civil Service (depending on Department). The SCS is made up of Assistant Directors, Directors, Directors General and, at the top of it all, the Permanant Secretary.
So in short, yes, you can end up as Permanant Secretary, but if that is your goal being on the FS won't change the need to be an exceptional candidate.