I always have 1.5 hours or more going thru ATL. 1/2 hr for boarding, 1/2 hr for flight delayed, and 1/2 hr to migrate thru the airport.
yes there is a train system linking all of the terminals in ATL. if you wanna walk it, it is 1,000 ft between each terminal (6,000 feet from terminal E to bag claim/ticket counter)