I'm going to have to add details and quotes later for this as I don't currently have access to the books, but I remember noticing some things I thought were foreshadowing for an underground system in the later AM Discworld novels. In Thud, a large system of underground tunnels with rails was developed by the dwarves and then abandoned, and the Moist von Lipwig arc all dealt with major infrastructure developments (banking, postal services, railways). Do we have any evidence that Terry Pratchett had plans for, or even started writing, a book concerning the development of an underground railway system for Ankh Mopork?
(Before asking I Googled this and noted that someone over on the LSpace wiki came to the same conclusion, but the page doesn't cite any sources from the Pratchett estate.)