look what you say is correct. but there will be some Technical issues where we cant comment on IRI. Example Zone changing point they have to change Drivers also. if driver get late train will run late.so Slac is compulsory and blasting a train in entire route in same speed is not possible even for RAJDHANI's and Shatabdis. on road we have side tacks and can over take. But in trains it is not possible boss.