It's not a daft question DBT and you have already guessed the answer. The orange cantrail stripe (or other colour if the vehicle iis already orange) indicates the maximum height that personnel are allowed to climb because of the OLE. I have read that they do not have to be painted on units that never leave the Southern third-rail system but I can't say for sure.
This withdrawn document gives some details: (I can't find the current one
)
http://www.rgsonline.co.uk/docushare/dsweb/Get/Rail-42837/rt2181_1.pdf