(1) DD has become a bad experiment for IR as well as public due to myopic vision of IR.
(2) AC DD coaches (with 128/120 seats against normal acc coaches with 71/75seats) were developed to increase the carrying capacity and revenues for IR, but what has happened was exactly opposite.
(3) Normal ac coaches of highly patronised full length trains running on IR, should have been replaced by the newly developed AC DD coaches which could have increased the carrying capacity as well as revenue for IR.
(4)...
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Instead IR embarked upon the new experiment of introducing ac DD expresses (for maximising the political mileage by way of new class of train).
(5) HWH-DHN ACDD was the first one and operated with just 7 passenger coaches and that too were not fully booked. So the result is the ACDD train capacity with less than 900 passengers capacity as against 22 coach normal train with over 1600 passengers.
(6) Another mistake was to keep the fare of AC DD same as normal ACC fare. The benefit of the economies of scale has to be shared with passengers, i.e. ACDD fare has to be a bit lesser than normal ACC fare. This is very much justified as AC DD coach/chairs are not as convenient as normal ACC coach/chairs.
(7) Now AC DD is no man's baby. IR don't like it for poor patronage & losses, where as people does not like it for inconvenience as compared to normal ACC (for same cost of the tickets.)
(8) As regards the special value, the reason is AC DD is for common man having regular acc fare, where as Shatabdi is for elite class with a base fare 20% higher than normal acc fare.