I was going to utilize A-C for "Idle Power" and an IR sensor to turn that off and turn on A-U. I have KW and ZW transformers and run 99% conventional.
Rob,
Your proposed method for holding track voltage low but not completely off, so that lights and sounds remain on during the stop, and raising it to restore motion is unique compared to most but appears to be quite reasonable. You're using the IR sensor as a relay in addition to its being a sensor.
The question with this approach, and a number of others, is how do you simply and easily switch back to motion, "undoing" the stop. Some sort of restart event is necessary.
If you want it to be a timed stop then this event is that the IR sensor needs to time out and switch the power feed back over from A-C to A-U.
If you want it to be manual, i.e. initiated by you, then the IR sensor needs to see ambient light again, or be tricked into thinking that it's seeing ambient light again (prior to this it's been blocked by the train), in order to do so.
These are the two main options if you're using the IR sensor as a relay.
There may be another. These others are usually not obvious but require a way to use the relay contacts and sensor in a novel way to accomplish your goal.
Before proceeding, is timed operation ok or do you want to use some other event to release the stop?
Mike