Manchester-Preston rail electrification delayed again
Old mines have been blamed for further delays to electrification works between Manchester and Preston.
Construction on the link first began in 2015 and was due to be finished by the end of 2016. This was later put back to the end of December 2017.
But "poor ground conditions... from old, known and unknown, underground mine workings" were delaying progress, Network Rail said.
The electrification is due to be complete in 2018, a spokeswoman added.
She said they were "delivering improvements to northern railways not seen since the Victorian times" but it was "taking longer in practice than we had hoped it would".
"One unforeseen factor adding time to our programme are the poor ground conditions we face from old, known and unknown, underground mine workings."
Network Rail believe "numerous locations" along the route - which is nearly 180 years old and one of the busiest in north west England - are affected.
It said engineers were "working hard" to solve the problem but rail journeys would continue as usual.
Construction works have already taken place at Bolton and Chorley stations in preparation for faster, more frequent electric train services, Network Rail said.
The route's electrification is part of the £1bn Great North Rail Project to upgrade links across northern England by 2022.