The "Icon Tray" here is actually the "Icon Bar" and discussed so widely it's the name of the main Risc OS forum to this day.

Pipedream always was spectacularly odd, even at the time.

Author here, thank you for pointing that out. I don't know why "Tray" was so stuck in my mind; the vernacular of the OS never quite embedded itself in my brain. I've updated the article to "Bar."