RudyB:
What is the 'factual faiult' you mention?
That one shouldn't power servos from the Arduino 5V as a matter of good practice... ok, maybe not factually incorrect as long as it's a small servo but even micro servos can draw 7-800mA. Both the servo sweep and knob examples say to do that... as soon as one moves to more than one and / or a bigger servo there's not enough current.
(I didn't say not to power an Arduino into its 5V pin, I said not to power anything substantial out of it.)
I like your idea of making your material "Arduino in a model railway world": that makes perfect sense for your audience, who do not want to get bogged down in the arcana of electrical engineering, but do want to get their model railways to work safely and reliably without too much fannying about.
Coincidentally, Robin2 had this to say to me yesterday:
Robin2:
I use them [nRF24L01's] for radio control for model trains. I have made some units for my model railway club that take the place of wired controllers so folk aren't tripping over the wires. And I have some installed in my own trains for battery powered radio control.