The growth of traffic in the early years far exceeded expectations. By 1866 that is, in only 12 years, the railway was carrying 4 million passengers and 800,000 tons of freight.
 
The single line was proving inadequate and the trading community began to mount pressure on the railway company to double the entire length. It would have cost about 6 million pounds, apart from the cost of additional rolling stock. There was no certainty of raising this capital in England where over 99% of EIR’s shares had been sold. The company, therefore, adopted the policy of ‘gradually doubling the line at convenient places from time to time and gradually stocking it.’
 
In line with this policy, it was decided to double the portion of the line from Luckeesarai to Allahabad. For the line below Luckeesarai, it was decided after considerable debate that it would be preferable to lay a shorter route than to double the line.
 
This 252 miles long line, connecting Raniganj with Luckeesarai would reduce the lead for coal towards the north by 100 miles. Coal for Upper India could now be conveyed directly, instead of a having to move 50 miles eastwards to Burdwan, before moving towards its westbound destinations.
 
Moreover, the new line would erode the preference Bombay port had been enjoying over Calcutta for imports from England because of its proximity to Upper India.
 
Construction of the line was slow and it was opened only in 1871. Initially it was called the chord line but as it began to carry more traffic, it was designated as the main line and the original line became the Sahibganj loop.