Mark 4 Coaches...two more unusued features spring to mind:
*The Crew Doors in the SV (Service Vehicle which contains the Cafe Bar and some Standard Class seats).
My understanding with these, after one came open while the train was in motion, in the early days of Intercity 225s, they became isolated from the door circuits. They almost re-instated them during the Project Mallard refurbishment at the turn of the 21st century.
*Automated announcements:
Well...I say "unused", but they do occasionally get used by Train Guards to remind passengers that ticket examinations are taking place. It uses the same voice that alerts the crew to the "Disabled Passenger Alarm Operated!".
Although there are recorded messages for station arrivals in the system, it's not automated as such, because they had to be manually activated each time, as opposed to GPS or wheel rotation! Unsurprisingly, I suppose most guards didn't bother using them in the Intercity days, and just did announcements manually, like they continue to do today.
Apparently it's known to say "Welcome to Intercity!" or "Thank you for travelling with Intercity!"