Because the Night Mail cache stopped working. The Night Mail starts off with, you need to identify the poem (pretty easy). Then you have to run a railway, controlling it over the internet, and watching it with a web cam - that lets you gather some information that gives you the starting point for a night cache, where you follow the trail of fire tacks, while night trains pass fairly near by.
Someone emailed me to say it wasn't working. When I had a look, sure enough, it wasn't. Some checks with a multimeter soon revealed that the 16 volt Hornby transformer wasn't working any more. So I replaced it with a standard 12 volt one, and that gives enough power for the train to run. Meanwhile, I made the amazing discovery that I could get a replacement transformer for just £4.00; I was expecting to be stung for several times that much.
The Night Mail