British Transport Police officers attended Braidhurst Viaduct in Motherwell after a report that items had been found on the line around 6.20pm on Tuesday night.
The track to the rear of Braidhurst High School was damaged but police declined to say what the objects were.
BTP officers are investigating.
Detective Inspector Kevin McCormack, the officer in charge of the inquiry, said: "I am sure someone in the local community will have information which would help identify those responsible for such a reckless act of vandalism.
"I am anxious to speak with anyone who was in that area at the time of the incident and who may have seen people acting suspic-iously on the railway line to contact BTP as a matter of urgency."
British Transport Police would not confirm what the objects were.
ScotRail services between Motherwell and Bellshill were disrupted.
A points failure due to the suspected vandal-ism led to delays at cancellations on train services in the Mother-well and Bellshill area on Tuesday from 6.40pm.
A ScotRail spokes-woman said: "Services returned to normal at around 8.30pm."
In August, the Evening Times told how a train hit a shopping trolley which had been thrown on to the line by vandals.
British Transport Police have launched an appeal after a Balloch-bound train struck the trolley on the railway line in the Dalmuir.
Detectives are not linking the two incidents.
British Transport Police can be contacted on 0800 40 50 40 or information can be given anonymously to Crimestoppers on 0880 555 111.