Here's a link to a 2007 NY1 story which quotes an NYCT official as saying that it is permissible to ride the #6 train as it goes around the loop track. I don't know whether the policy has changed since then, but AFAIK, the automated train announcements referred to in the story have not changed.

http://www.ny1.com/Default.aspx?SecID=1000&ArID=67432