Rail passengers are facing disruption after overhead power equipment on a busy line suffered significant damage.
Customers have been warned to expect changes to services and some delays throughout Wednesday as engineers work to assess and repair the problem in Glasgow.
The damage was discovered at 4.46am when the driver of a train that was clearing the line ahead of the start of service reported his windscreen had been damaged by an object hanging from the overhead line next to a tunnel near Finnieston.
Investigations revealed an insulator, which regulates the current, had come loose and there was significant damage to the electrical line.
It is not yet known how the insulator came to be damaged, though it could be related to the weather or a bird strike.
ScotRail declared a major incident and put contingency plans in place.
It comes as Transport Minister Humza Yousaf makes a statement to MSPs on improving rail services after criticism of current ScotRail operator Abellio.
A ScotRail Alliance spokesman said: “Significant damage has been caused to our overhead power equipment near Finnieston and engineers are on site working to assess and repair the damage.
“Extra staff have been deployed to stations to assist customers with their journey. An alternative train plan has been put in place that, although meaning changes to services and some delays, has kept people moving during the peak period.
“Due to the nature of this incident, it is likely that the alternative train plan will be in operation throughout the day. Customers are advised to check before they travel and to give themselves more time to complete their journey.
“In addition, other travel options - including ticket acceptance on First buses and the Glasgow Subway - have been put in place.”
Some services have been cancelled while others are starting and terminating in different places, or are making additional stops.
ScotRail said if the incident continues into the evening peak, people attending the Celtic v Barcelona Champions League fixture will still be able to travel to Celtic Park from the city centre by train.
However, rather than taking the Glasgow Central service to Dalmarnock, they will need to travel from Glasgow Queen Street to Bellgrove.