Rail passengers on the trans-Pennine route could face "significant disruption" for five years during its £2.9billion upgrade, a senior transport official has said.
A letter from Rob McIntosh, a route managing director at Network Rail, to the Transport Secretary warns of the level of problems the project could cause if the upgrade is completed by 2024.
It reveals there could be route closures for up to 39 weeks a year, longer journey times and fewer seats on trains during the work between 2020 and 2024.
Mr McIntosh also said once complete, the work would eventually "transform" passengers' experience between York, Leeds and Manchester through reducing journey times, boosting capacity and revamping stations on the route.
But he warned it "cannot be delivered without significant disruption" due to having to upgrade an old railway in the "inherently challenging" landscape of the Pennines.