Tamilnadu (121/122) was bifurcated from AC Southern Exp & sped up by prefixing 1 in train number of AC Southern Express (21/22)!!
This is another great example of two trains which once run together!! Original slot was retained by Dakshin & hence Dakshin will be taken as older version & original AC Southern Version!!

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TN was a completely new concept
It was not successor of any train !

And reason for dakshin shifting to HYB was
/blog/post/5084513

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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!!

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Kindly read reply to your highlighted link to know if Dakshin Exp was shifted to Hyderabad or it run MAS-NDLS with slip portion from Hyderabad in 1966 named 21/22 Southern Express/Deluxe!!
Tamilnadu concept could be possible only after track upgradation (in 1966 Southern Exp runtime cut short by 4.5 hours after complete route got diesel Loco!!) Incidentally Dakshin/Tamilnadu were first complete Diesel E2E SR train!!

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