British Transport Police (BTP) said sections of cable were found to have been damaged and stolen beside tracks in Cumbernauld at around 4:30am.
The incident halted trains until 10am, with passengers transferred to buses, although knock-on disruption continued later into the morning. A second line, between Cumbernauld-Motherwell was also affected.
A ScotRail spokeswoman said: “We apologise to customers for any inconvenience this morning as a result of this vandalism.”
The cable thefts comes a month after a series of vandalism attacks on rail signalling across Glasgow which police said had caused “extensive disruption to journeys”, with hundreds of trains delayed or cancelled.
Network Rail is offering a £10,000 reward for information about the incidents, which happened within eight days of each other at Queen’s Park, Yoker, Nitshill and Jordanhill.
They are believed to have been carried out by “someone with a working knowledge of the railway”. Police patrols have been stepped up and a Network Rail helicopter drafted in.
A BTP spokesman said: “The nature of the faults at each location and the timing of the incidents has led the rail industry and BTP to believe they could be linked and represent a deliberate attempt to disrupt rail services across Scotland’s largest city.”