Currently my TH 7 account is upgrading a barracks for a second dragon barracks.
In the previous version of the game, having a second dragon barracks meant
that you only had to log in twice to train a full dragon army (as opposed
to 4 times). With the new quick train feature, you can create 2 full dragon
armies by queuing 20 dragons. The goal for these statistics is to see if there
is any real advantage in training times by upgrading in the current game, as
well as trying to nail down how quick train and # of barracks work.
One dragon barracks: dragon trains in 12 min.
Three active barracks: giant trains in 40 sec.
One dark barracks: minion trains in 36 sec.
Four dragon barracks: dragon trains in 3 min.
Four active barracks: giant trains in 30 sec.
Two dark barracks: minion trains in 18 sec.