Thanks, Steve, just a response makes me feel less lonely!

In the temporary absence of this forum I asked on two other forums but the silence was deafening.
Those two links to videos are interesting - I did learn a couple of new tricks but didn't find the answer I was looking for except that Matt did say briefly that you can exclude passengers from a specific area with occlusion boxes, but then brushed it away by saying he had never been able to make them work.
If you know me by reputation I am a bodger of modern routes, turning them back to steam-era. The screen shot comes from my 1960s mod of Keith M Ross' WCML over Shap route, so opening that route in the editor and look at the Penrith Up Platform you should see them - but you don't! It looks as if they are only visible if you create an invisible platform, and instantly break the invisible platform's abuilities.
The only occlusions I have come across are for tunnels, Tunnel Occlusions are found under Object Tools > Miscellaneous but no reference to Platform or Passenger Occlusions there.
But . . . a little exploration reveals:
1. There are two "Tunnel Occlusions" underground below the platform - I removed these. In theory they were only there to stop rain from showing under the canopy.
2. I then removed and replaced the invisible platform. The yellow boxes still show. I note they only show if the invisible platform is close enough to conflict.
3. Even when an invisible platform is laid clear of the yellow boxes, no passengers show
I'm currently defeated on this. I still suspect something is coded in somewhere creating those boxes but I cannot remove them.
Still, I have learnt a little - thanks.
John