The rails are said to have been introduced into churches in the ninth century. They separated worshipers from the church's sanctuary, reminding the people that this was a sacred area, and kept animals away form the sanctuary.
When the faithful began to kneel to receive the host, Capuchin author, Fr. Edward Foley, writes that these rails "seem to have developed from the practice of spreading a cloth for communicants kneeling at the altar."
There never was a Vatican ruling that prohibited the rails ... just as there hasn't been a Vatican ruling to reinstall the rails either.
In church of other denominations, Communion rails have remained popular, although there is no requirement that they be installed.
- Little White Book, Diocese of Saginaw