A spokesman for the company said it was “closely monitoring demand” and bringing in additional carriages when possible.
But he added it was “challenging as we have a limited number of carriages in our fleet, which are needed to run more than 2,650 services a day across the North of England”.
“Limited capacity at Newcastle station can also prevent us from running longer services at certain times of the day,” he added.
“We are exploring all available options and working with industry partners to address these issues.”
The number of journeys on the line has been about double the number expected despite two stations – Bedlington and Northumberland Park – still not having opened.
Its success has prompted discussions on an extension, with Newbiggin identified as a key destination.