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The Track Controls button is only on the iPhone 6Plus or 7Plus available, see this Help page;
http://help.apple.com/garageband/iphone/2.3/index.html#/chs157aec9e
On an iPhone 7 Plus or iPhone 6 Plus, the Track Controls button opens the the Track Controlsfor the currently selected track.
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On an iPhone 7 Plus or iPhone 6 Plus, the Song Settings button lets you view and edit song settings, including metronome, tempo and key settings. You can also open GarageBand Help.