Police have arrested an 18-year-old man and a 14-year-old boy in connection with Sunday's disruption on the MTR's East Rail Line.
It was caused by a 17-metre-long metal railing being thrown onto a power line, apparently from a footbridge.
The incident happened near Tai Wo Station in Tai Po.
Early morning train services on the line were disrupted for about an hour.
Police classified the case as criminal damage.