Subterranean railway fantasy

A sort of dystopian fantasy which involves a strange subterranean railway leading to a strange world... can't remember much more detail. It's a relatively recent book. As far as I can recall it has a strange one-word title, possibly beginning with S or P (or possibly not...)

Adult or children's fiction? If one-word title, perhaps unlikely to be The horn of mortal danger by Lawrence Leonard. What was the earliest year you recall reading it?

Adult fiction. I recall partially reading it just two or three years ago and not finishing it as it was quite heavy going, but I'd like to revisit it now.

Kinda sounds like Neverwhere, but that book is not heavy going in the slightest.

Palimpsest? I think that counts as a strange one-word title...

>5 rarm: Thank you! I'm pretty certain that's the one!