The Transatlantic Conspiracy — G.D. Falksen

At the dawn of a reimagined 20th century, one girl must become the reluctant symbol of a new world.
 
The year is 1908. Seventeen-year-old Rosalind Wallace's blissful stay in England with her best friend, Cecily de Vere, ends abruptly when her father books Rosalind on the maiden voyage of his fabulous Transatlantic Express, the world's first railroad to travel under the sea. Rosalind is furious. But lucky for her, Cecily and her handsome older brother, Charles, volunteer to accompany her home.
 
But when Charles disappears and Cecily and her housemaid, Doris, are found stabbed to death in their state room, Rosalind finds herself trapped undersea, in a deadly fight to clear herself of her friend's murder and to thwart a sinister enemy.