The Furthest Station
Book #5.5 in the Peter Grant series (2017)
While ghosts on the London Underground are very common occurences that nobody really thinks much about, when the Metropolitan Line has been overrun with seemingly aggressive and are frightening the commuters, PC Peter Grant, working both for the Metropolitan Police and The Folly, is asked to investigate. After all, being a wizard and all that, he should easily find the source of the unrest.
So Peter and Jaget Kumar, his counterpart at the British Transport Police who initially asked Peter for help, have to check out the Metropolitan Line of the Tube train where numerous people noticed ghosts, but who are, strangely, unable to recall any of it later on.
So now Peter and Jeget have to run against the clock and find a witness who has encountered ghosts…before she loses her memory of it as well.