A New #9 Question

Here's a new question for Steven and Shane.
Thanks to being leave home early on a business trip, I was able to stop by the Loop last weekend. I wasn't able to ride because of another sellout of the 5-car train, but the visit was worthwhile nonetheless -- #9 put on an absolutely spectacular show.
On to my question: while I was trackside, I observed a mid-day refueling and watering in Silver Plume. As part of the refueling and watering stop, the crew disconnected an external air line. It looked to me like the crew used an external source of compressed air to top off #9's air tank. Did I totally misunderstand what I saw? If I did, what was I seeing?
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Chris Webster
[www.speakeasy.org]