As to redundancy. How much do you build in. Do you back up the backup ? What happens when the backup fails ?
All pretty obvious stuff: you have one backup and once one of the two fails you book in maintenance but the unit doesn't have to be pulled out immediately. Ideally you do this for everything up to and including the driver. In reality you only do it where the cost/benefit pays off, but a backup PA system doesn't sound like it should be very expensive. DOO only makes it more worthwhile, of course.
The point made about how reliable these systems is is, of course, important and I don't have any idea what the mean time between failure is. My experience is that PA systems have improved but even on the still-quite-new class 800s I ride to work and back every weekday on, it's not that unusual to have mysterious pops and crackles which make the message hard to understand. Low volume is a fairly common problem too.
Definitely better than anything SWT could muster in the 90's when I lived in Brookwood, though. Sometimes the carriage would dissolve into laughter as some completely garbled set of pops and whistles gibbered at us for a minute or more. It was unusual to have a journey without at least one lost announcement.