BTP officers were called to the railway line near Dyffryn Estate, Newport, after Network Rail reported a loss of power to signals in the area.
The incident was reported to BTP at 12.02pm.
Officers attended along with engineers from Network Rail and discovered damage had been caused to lineside signalling equipment.
The incident led to a number of trains being delayed or cancelled, causing disruption for passengers and the rail industry. The lines were fully reopened at 2.25pm.
Enquiries are ongoing to trace those responsible for the damage.
Anyone with information which they believe can assist, should contact British Transport Police on 0800 40 50 40, or the independent charity Crimestoppers on 0800 555 111.
In all calls, please quote incident reference 250 of 14/12/2012.
Icy by night, chilly by day - brace yourself for a cold week ahead.
High pressure keeps the weather chilly by day, icy by night, with an increasing risk of freezing fog.
High pressure this weeks means mainly cold bright days and icy cold nights.