A three-day rail maintenance program will mean trains will not run from Kooragang to Narrabri from Tuesday to Friday.
The works started at 6.30am on Tuesday morning and will finish at about 6.30am on Friday.
About 1300 workers will carry out maintenance and enhancement works along the Hunter Valley rail network, involving different 125 projects.
ARTC group executive for the Hunter Valley Jonathan Vandervoort said maintenance works along the network included re-signalling at Thornton, a level crossing upgrade at Army Road at Glenridding, and track reconditioning work at Quirindi.
"The Ardglen Tunnel will also be re-sleepered and re-railed. This is an interesting job as it is a heritage-listed tunnel and we have to take those considerations into account while undertaking the works," Mr Vandervoort said.
"We will also take advantage of the opportunity to undertake a number of works including removing ballast and improving drainage on this section. With tunnels you need to be very efficient as the conditions are difficult to work in and to you need to be very organised to get all the works done in the allotted time."
Mr Vandervoort apologised for the inconvenience and said he appreciated the "patience and cooperation of motorists, commuters and customers right across the maintenance program".
"We also ask people in communities close to the rail corridor to be cautious during the shutdown period and keep an eye out for increased vehicle movements in and out of work sites," he said.
With the shutdown affecting Hunter Valley train services, alternative bus services will be provided, and details and schedules can be found at the Transport for NSW website.