From the horse's mouth.
Just pinged Steve poerschmann, who runs the train. Sounds like delta should pin him too.
"Normal operating hours for The Plane Train are 5 am to 1:30 am. We can offer limited extended service if air carriers notify us by midnight and there are no scheduled APM maintenance conflicts.
Thanks for your inquiry.
Steve
Steven E. Poerschmann, C.M.
Director, Aviation Transportation Systems
Hartsfield-Jackson Atlanta International Airport "