I'll do it from two directions. Prose, first. When I opened the book I found the text Pratchetty, but different -- the rhythm was all kiltered. Too many little phrases and parenthetical asides, the "indeed" and the "as it were" and the "so to speak". It's not clean. The Pratchett I know can run you through with a sentence and make you laugh at the same time, and do it again twice a page. This isn't that.
Pratchett can no longer type. "After falling out with his keyboard, he now talks to his computer", the blurb dryly notes. I remembered this and went back, and yes: this is spoken Pratchett. That's the balance. (I heard him speak once, at a Worldcon, years before his illness.) I don't go for audiobooks, but I think this must work best as an audiobook; that's how it was composed. The printed text is a word-for-word translation from voice; it shows.
Then, the story.
It's soft. I can't excuse that. This is the book of how the railway came to Ankh-Morpork, and the railway comes, and there really isn't a lot more to it. The story steams along and everybody is along for the ride. Oh, there's resistance and an antagonist (conservative dwarf priests) but they don't provide much resistance. The good guys wipe the floor with them. It's easy. Pratchett has never done easy. Even when he's doing pure farce, the protagonist is (...Rincewind is...) terrified and that has weight. Victories cost. This book has no cost. Even the conservative dwarfs just wind up in jail, except for the ones who don't surrender and get killed by the good guys, and that has no cost either. I'm not happy.
Conservative dwarf priests are an easy target to begin with. Look, I get plenty irate about conservative real-life priests. In the middle of reading this book I read articles about the World Vision affair -- you can google it -- a bunch of conservative evangelicals rose up to defend their principles, and their principles turned out to be "Better a thousand children starve than one Christian stop hurling shit at gay people." They won, too. (For now.) But that's all we get of the dwarf grags. They want dwarves to stay dwarfish, and that means blowing up clacks towers and steam locomotives and (eventually) being beaten up by victorious good guys.
Look (again); I know Pratchett has always loved the absurd, the over-the-top evil villain. He does lots. More than one have been closed-minded priests. But there should be... empathy, if not depth. Even as the villain is utterly crushed, we should feel sorry for the fragment of him that is in us. That's what Pratchett does. This book doesn't carry it.
It's not the book I wanted. The whole story of Vetinari's Undertaking, the modernization of Ankh-Morpork, has been about Vetinari's death! Vetinari is one of the two Discworld characters who are absolutely irreplaceable -- when the Patrician dies, Ankh-Morpork collapses. Vetinari knows it and will not abide it, and book by book he's been making himself obsolete. He's been doing it since he sobered up Sam Vimes; he's done it step by step in the Moist von Lipwig subseries. This book could have capped that, written from Pratchett's knowledge of his own fate. It didn't. I don't know if I can forgive it.
(The other irreplaceable character is Granny Weatherwax, who -- in this book packed with cozy cameos -- doesn't appear at all. Hopefully Pratchett feels he's tied that off with the Tiffany Aching books. I don't know if I could stand a soft Esme Weatherwax story.)
(Okay, yes, Death is irreplaceable but he doesn't count.)
This is a crappy review and I don't like writing it. Pratchett has not succeeded in making himself obsolete, the indomitable bastard. (Although his daughter is kicking ass and will hopefully carry the Discworld torch, along with many others.) I know he's still publishing older work; I don't know if he's still working on new Discworld stories. I don't want to say that my expectations have dropped, but they have. I don't want to tell him to leave it alone, and I won't. I value both refusing giving up and knowing when to give up. There's no pat answer here.