North Wales Police are investigating after four cast iron benches were found on a railway line.
The benches were discovered on the tracks of the Welsh Highland Railway near Beddgelert station on the evening of Wednesday, 9 February.
No damage was caused to the railway line or the trains which run on it.
Police are appealing for information on 101 (if in Wales) or 0845 6071001 (Welsh language line) or 0845 6071002 (English language line).
As the benches are heavy, officers said they believe it would have taken a group to move them.