Even though MARTA has released data to Google Transit, it has not made data available to the public as far as I know.
If you can find GTFS data for download for Atlanta, let me know. Otherwise contact MARTA and ask them to publish the data they give to Google: http://www.itsmarta.com/contact-us.aspx
Please ask MARTA to give you a link to the data (GTFS format).
I received a reply that said they released information to Google transit. What more do you need from them?
josh york commented
Atlanta is a huge city and I'm surprised that it isn't already on the list.