It's part of the livery of almost all* UK trains: The orange stripe at cant rail level is to denote the minimum safe distance from overhead electric wires.
On some liveries with an orange element (Or pink, in the case of 66587!) the stripe is white in order to contrast, and GNER had dispensation for the colour used to match the red bodyside lining band.
*Some third rail DC only units, South Western class 444s, 450s and 458s, don't have one.