Dick Simnel is inspired -- by his father being turned into a cloud of furnace parts, flying metal, and pink steam -- to invent the locomotive. He names her Iron Girder. This being Discworld, she's a bit more than her earthly counterparts. She is self aware and will defend her existence, as a would be saboteur learns.
Remember the pink steam? Simnel manages not only to avoid that, but to sell Sir Harry King on steam locomotives. King sells Lord Vetinari, who runs Ankh-Morpork, the big city on Discworld, and Vetinari appoints Moist von Lipwig (of Going Postal and other books) to ramrod the project. Vetinari want to go to Bonk Schmaltzberg to confer with the king of the trolls, without spending weeks on the trip.
Certain hard-cases among the dwarfs don't like this, or anything not of dwarf tradition as they see it. So construction is underway on the big line, with a shorter one to Quirm (which resembles France as seen from England) to bring in fish. Yes, Ankh-Morpork is on a river, but you really don't want to eat anything out of there.
Then there are the clacks towers (semaphore signaling system), and a lady named Adora Belle.
Did you know that goblins are very talented with machinery? And that Lu-Tze is the master of deja fu? That mastery of the sliding rule is essential to avoiding the pink steam? (Google "slide rule", guys.)
The map alone is worth the price of admission: Pseudopolis, Aix en Pains, Para Mount, Dontgonearthe (don't ask!), Bad Schuschein, Seven Bangs, Big Cabbage, and the Effing Forest.
In case you are a Pratchett virgin, Discworld is a flat earth, which sits on the backs of four elephants, which stand on the back of the great turtle Atuin, which swims through space from where it was born to where it will mate.
The big bang theory. That sound was me ducking.
Look out for Of the Twilight the Darkness, and don't shorten his name. It's insulting.
In which Moist von Lipwig discovers an unexpected talent for combat, and Drumknott learns that there is more in life than figures.
It was with great regret that I heard that Pratchett has died. I hope he left several manuscripts that will be published shortly.