Ah that explains why the tests failed... I need to start trying both activators and buttons. I also wish this was in the wiki... Maybe I'll have to create an account.Rail dockers emit a high/on signal when they dock.
When a rail docker undocks it emits a low/off signal.
If a activator is placed next to the docker it will receive the high/low and change state to match if it needs to.
So a activator will turn on when it docks and off when it undocks, but only if it is active.