Welcome to Trainz and to the forums!
The AWS system is engaged automatically, I think, and is controlled in the session setup through an AWS rule.
The only time you need to acknowledge a signal is when it's something else other than green. If you fail to do so, I think your session ends. It's been awhile since I've done the built-in sessions so I might be wrong on this. I instead spend most of my time working on my own routes, or testing those by others.