The lines run in an east–west direction between the Ganges and India-Nepal border, covering northern Bihar west of the Kosi river and eastern Uttar Pradesh.

In his book The Indian Empire, Its People, History and Products (first published in 1886) W.W.Hunter, says "The Tirhut State Railway with its various branches intersects Northern Behar and is intended to extend to the Nepal frontier on one side and to Assam on the other.

The 270 km (168 mi)-long Tirhut main line from Katihar to Sonpur was developed in stages between 1887 and 1901.

[2] The lines were converted to 1,676 mm (5 ft 6 in) broad gauge in phases starting from early 1980s.

Gauge conversion of the 268 km (167 mi) long Jainagar–Darbhanga–Narkatiaganj line that was started in 2011 was completed to Raxaul in February 2014 and to Narkatiaganj in 2017.

The construction of the 2 km (1.2 mi)-long Rajendra Setu in 1959 provided the first opportunity to link the railway tracks on the north and south banks of the Ganges.