In this case the rails are already there but unused.
That is also several strike against this. Those rails exist but they are all in really bad shape as they were nearly universally used without maintenance until it was no longer feasible. They are also generally in bad areas where there isn't much need for more transport - we already have roads in good shape (to run a bus on). The only thing this has over a bus is you can run them fully automated - which isn't enough IMO.