Transit Divisional Response Unit members are appealing for witnesses after a train hit a shopping trolley which was thrown onto the tracks at Malvern Railway Station earlier this month, Victoria Police say.
Police believe the trolley was thrown from the Glenferrie Road overpass into the path of an outbound Pakenham train about 10.45pm on 2 September.
The train struck the trolley which then became wedged under the first carriage.
The train was faulted and a technician was called to attend, causing a major disruption to services.
Six to eight teenage boys were seen heading north along Glenferrie Road shortly after the incident.
The boys were wearing baseball caps, hooded tops and backpacks.
Police believe the teenagers may be able to assist them with their enquiries.
Members are investigating and are appealing for anyone who saw the incident or any suspicious activity in the area to come forward.
Anyone with information is urged to contact Crime Stoppers.