The Southern Express/Dakshin/Deluxe always connected MAS & Hyderabad to New Delhi till Tamilnadu Bifurcation!! 21/22 was main portion while Hyderabad was Slip of two coach in 1966 (attachment pic 2)!!
Before Tamilnadu inception, track wasn't feasible for faster run and was into improvement (Sped up past diesel Traction)...which kept improving while Southern Express plied MAS/HYB/NDLS!!
And then Slip Southern Express become full fledged Dakshin Exp & as two trains can't have same name, main portion was named Tamilnadu!!