Part of a ScotRail train fell off and hit signalling cables today near Glasgow Central causing around 100 cancellations.
The highly unusual incident involved a prop shaft dropping from a class 156 diesel train near Shields junction on the Glasgow-Paisley line in Pollokshields just after 6am.
An industry source said it had also caused a total of more than 21 hours of delays to trains coming in and out of Scotland's busiest station.
The location of the fault near the Shields depot at the start of the morning peak period compounded the problem because it prevented trains from leaving.
This caused significant knock-on disruption after repairs were completed at 6:50am.
The source told The Scotsman: "Today's disruption into Glasgow Central was caused by a component falling off a ScotRail train.
"A prop shaft fell off a 156 and damaged signal cables.
"Network Rail engineers had to warn ScotRail it had happened and get train taken out of service.
"Has caused over 100 cancellations and over 1,300 minutes of delay."
Network Rail said more than 60 trains had been cancelled by 9am alone, with 15 hours of disruption to others by then.
ScotRail said: "The detail on the 156 is being looked at as the probable cause."
It said services from Glasgow Central were subject to delay and alteration due to a fault with the signalling system.
Network Rail engineers attended and fixed the fault to get trains back to normal.
A ScotRail spokesperson said: “We are sorry to customers who were delayed as a result of a signal fault today.
"Engineers attended the incident and worked as quickly as possible to fix the fault and get services back to normal.
“Anyone delayed by 30 minutes or more is entitled to claim money back through our Delay Repay guarantee on our website or mobile app.”
Later, passengers in Fife were delayed after a power problem affecting signalling between Dalgety Bay and Burntisland.
Services were disrupted between 1:30pm and 6pm.