It is a retro-reflective strip ( Reflects light back in the same direction as it is incident). It is placed two masts before signals, generally in fog prone areas, but can be placed in other places too. It is a visible sign that tells the LP that there is a signal up ahead. In this pic you can see the signal hidden just behind the mast with the sigma strip. As the signal is on a curve, the sigma strip will be used to indicate that there is a signal a short distance away.