As train tracks spread across 19th- century America, so did a clandestine network - a secret movement to convey runaway slaves to freedom, dubbed the Underground Railroad.

This is transformed with a touch of fantasy into a literal railroad in American author Colson Whitehead's bold, violent sixth novel, which features an underground locomotive that picks up fugitives from hidden stations in each state.

It is an innovative way to narrate a much-told history of violence which the present cannot afford to grow numb to, especially given rising racial tensions in the United States today.

The passenger with whom the reader rides the rails is Cora, a young outcast on a cotton plantation in Georgia who flees with another slave, Caesar, for the subterranean train.

As they make their journey north towards freedom, each stop presents fresh wonder and unexpected horrors. They are helped along the way by "station agents", many of them white people risking their lives. But hard on their heels is the infamous slave catcher Ridgeway.

The railroad invigorates the novel and it is a pity Whitehead does not make more use of it.

It is a shifting, fascinating system, constantly changing to elude discovery. Each train Cora takes is different, from a lavish 30-seat boxcar to an unstable cargo platform she must strap herself to or fall off.

The narrative has a tendency to go off-track. Its detours into the lives of minor characters can be insightful, such as that of Cora's grandmother Ajarry. But others, like the back story of a white South Carolina medical student, are distracting bumps on an otherwise gripping journey.

It is never explained how the railroad came to be. When asked who built it, its agents respond: "Who do you think?"

In this respect, the novel follows genre-bending slave narratives such as Toni Morrison's ghost story Beloved and Octavia Butler's science- fiction tale Kindred. They ground the fantastical in matter-of-fact realism: revenants, time travel and continent- wide subways are inexplicable, but so is the depth of cruelty that sustained slavery in the US for centuries.

At each stop Cora makes, she believes herself to be out of harm's way - but over and over, each refuge gives way to persecution and terror. Her journey parallels that of African- American history, plagued by the question of when a race of people can ever count themselves as truly safe.

If you liked this book, read: Beloved by Toni Morrison (Vintage, 2007, $18.94, Books Kinokuniya), in which a former slave is haunted by the ghost of her baby daughter.

Olivia Ho