Vandals ripped cable from a train track causing severe disruption for rush-hour commuters.
British Transport Police say a signalling cable was stolen at around 4.30am today in the Greenfaulds area of Cumbernauld.
The damage led to misery for passengers relying on train services from Glasgow Queen Street to Cumbernauld and Falkirk Grahamston, causing delays and cancellations for those heading in and out of the city.
Journeys between Motherwell and Cumbernauld were also cancelled or suspended.
A spokesman from British Transport Police could not confirm how much signalling cable, which runs alongside the rail track, was taken.
Today marked the second morning of rush- hour disruption on the train lines after major signalling problems at Glasgow Central Station threw services into chaos yesterday.
A ScotRail spokesman said today: "We apologise to customers for any inconvenience this morning as a result of this vandalism."
A Network Rail spokeswoman said: "Cable theft is a growing problem for the industry. It is not only a dangerous and irresponsible thing to do but it also disrupts journeys for the travelling public."
Glasgow Queen Street to Cumbernauld services were back up and running at just before 10am and Motherwell to Cumbernauld services recommenced at 10.10am.
As the Evening Times went to press, some stations on the Glasgow Queen Street to Falkirk Grahamston - between Camelon and Glasgow Queen Street - were still not being served.
ScotRail provided a replacement bus service for passengers travelling between Glasgow and Cumbernauld and from Motherwell to Cumbernauld.
Customers with valid ScotRail tickets were able to use their rail tickets to board First Glasgow buses to get to their destinations, at no extra charge.
A Network Rail spokeswoman said: "Regrettably signalling cable theft has caused disruption to services around Cumbernauld.
"Cable theft is a growing problem for the industry.
"It is not only a dangerous and irresponsible thing to do but it also disrupts journeys for the travelling public."