The line which u are mentioning is the Azimganj bypass line (indicated in the picture). It was constructed for directly bringing the Coal wagons for Sagardighi Thermal Plant from Rampurhat Marshalling Yard WITHOUT having any reversal at Azimganj,hence taking the shorter bypass route dedicated for freight.
ЁЯФ╢ Sometimes route diverted Mail/Express trains have also used this line,but no regular trains use this bypass.