if you really want to travel from mumbai to norther india like delhi amritsar etc you have trains like Paschim Express or Golden Temple Mail. they are sf trains and are basically meant to cater big stations and mumbai passengers. regarding this train , well you need some express for intermediate passengers to travel long distance , otherwise they have to board a passenger go to some big station and then catch some sf which is not always feasible option, so these sort of trains are needed not for mumbai - amritsar but to cater needs of smaller stations