SK concept is not flop, almost all Sk's are doing very well. Only flaw of Sk's train are even if it has very few halts but their speed are on par with Normal Sf.
Whereas Duronto provides speed of Rajdhani & also provide Rajdhani type connectivity to few non capital but very Important locations like Pune,VSPk which are not touched by Rajdhani.
Also provide Fast connectivity between Diffrent Metro cities.