I understand it is only approx 1 to 1.5 hours driving time between Austin and San Antonio compared with 3 hours Austin to Dallas-Ft Worth. However, I think if they connect Austin to San Antonio, they should have plans on Austin to Dallas-Ft.Worth.
http://tpr.org/post/san-antonio-austin-commuter-rail-not-dead-yet#stream/0
If the Texas Eagle is still around at implementation at least this will speed up that train further as it can cut some stations out between Austin and San Antonio and take advantage of the upgraded speeds on the line.