It's the cantrail stripe to indicate the point at which you shouldn't go above when under overhead wires. I think almost every train has it [1] even if it's a DMU or 3rd rail EMU as it can still operate under overhead wires.On a similar note and slightly off topic, why do trains have a thin orange stripe at the top of the TOC livery?
I noticed this years ago on the SPT livery's but assumed that it was just a line that was used to mask off an area for the Carmine and cream, however on a recent trip to Glasgow central Scottm and myself noticed it was on the Pendolinos as well.
[1] The one exception I can think of is the ex-Tube stock on the Island Line, but I'm sure there was another train I've seen without it.