It was hinted in "Thud!" back in 2005 that an Ankh-Morpork railways book might be coming up; this is it. I think this might have worked better as a short story than a novel; it takes a long time to get to the plot, and the railway stuff isn't really Discworld-y enough to be as funny or as subtle as I'd normally expect from this series.
ISBN 0857522272.