That is the coupler wire and one wire would have constant DC Positive voltage present. So it that arcs to the frame which would normally be carrying AC you can get arcing, and in worse case when return comes in contact with frame, ac power fed back on the PS-2 or 3 board and that usually means total damage to the board.
Coupler wires can be a source of that arcing if motion unaffected. And as others have said, if powered via trucks the red center rail another source if the insulation becomes damaged. G
