Composition of both trains will be same.....it will help in avoiding rescheduling/cancellation
For e.g. in winters if Pawan Exp arrives 5-6 hrs late and Kushinagar arrives on time/1 hr late...then rake of Kushinagar can be sent as Pawan Exp after PM......and the rake of Pawan Exp (which arrived heavily delayed) will depart as Kushinagar in night...