Emergency services are dealing with an incident on the train line between Portadown and Lisburn.
It is understood a train hit an object on the line close to the Knockmore Road this morning but Trankslink have not confirmed this.
No injuries have been reported and a spokesman described the obstruction as "minor".
A Translink spokeswoman said: "Translink can confirm that due to an obstruction on the line between Lisburn and Portadown, rail services have been disrupted from around 6.45am this morning.
"Bus substitution services have been put in place for local passengers between Portadown and Lisburn and Cross Border passengers will be bussed between Belfast and Newry.
"Disruption is expected until further notice."
#NIR obstruction 08:00 Belfast Ctl-Dublin Connolly 90 mins late Newry https://t.co/JLshA8m2zG
— Translink (@Translink_NI) February 4, 2016
@Translink_NI Anyone know what's going on with the train at our work at Knockmore Lisburn Ford pic.twitter.com/wiu6ksHo86
— Ross Scott Manley (@RossScottManley) February 4, 2016
The scene of an incident involving a train near Lisburn @irish_news pic.twitter.com/LNURyGFLto
— Mal McCann (@MalMccann) February 4, 2016
#NIR Due to an obstruction on the line a bus sub will be provided between Portadown and Lisburn. Thank you for your patience ^JC
— Translink (@Translink_NI) February 4, 2016