"The Barq's that is dispensed from Coca-Cola Freestyle machines is caffeine-free.[8] This is because the system uses the same concentrated, microdosed ingredient for both Barq's and Diet Barq's; the only difference between the two is the sweetener that is added."
http://en.wikipedia.org/wiki/Barq%27s
I saw this the other day wondering what happened to Ramblin' Rootbeer.