Also these zones like NR,WR,CR,CR were the zones from where maximum Shramik trains originated ( Maharashtra, TN, Keerala, Gujarat,Delhi ,Rajasthan,Haryana). The cost of sending the shramiks and then train coming back empty would have also added to cost. Zones like SER, ECR,ER always have advantage of Freight whereas they did not have to run a single shramik train if I remember correctly