The Red Signal (Musaicum Romance Classics). Grace Livingston Hill

When David Stevens, engineer on freight No. 5, snatched Hilda Lessing from the path of a fast express, he did not know that there was the making of a heroine in her slender frame, nor that fate was to entangle this simple child with himself in a web of plots and danger, from which she was to rescue them at the risk of her life. But some of the most breath-taking adventures of all fiction lie between the consummation of her happiness and the foiling of a diabolical plot threatening the safety of the nation.