Veruncheek, I found the Easter Egg!
Thanks to you, that goes without saying, because I wasn't aware there was an Easter Egg. I was more or less systematically clicking on the train master's papers, not really expecting to find it, when suddenly this really big Jordan Mechner appeared out of nowhere. Quite a shock.
Spoiler:The last page, the line with the words "movements" and "guides", click on the single "a" on the right.