The Furthest Station

There have been ghosts on the London Underground, sad, harmless specters whose presence does little more than give a frisson to traveling and boost tourism. But now there's a rash of sightings on the Metropolitan Line and these ghosts are frightening, aggressive, and seem to be looking for something.
Enter PC Peter Grant, junior member of the Metropolitan Police's Special Assessment Unit, AKA The Folly, AKA the only police officers whose official duties include ghost hunting. Together with Jaget Kumar, his counterpart at the British Transport Police, he must brave the terrifying crush of London's rush hour to find the source of the ghosts.
Joined by Peter's wannabe wizard cousin, a preschool river god, and Toby the ghost hunting dog, their investigation takes a darker tone as they realize that a real person's life might just be on the line.
And time is running out to save them.