Tunnel 6

When Geraldine Halloran arrives at the Central Pacific end of track in the winter of 1866, she's resolved not to be terrified. After all, she has a right to be there. She was hired on as regular telegraph operator, though they thought they were hiring a man. She's the only other woman at Tunnel 6, except for Mrs. Strobridge, the superintendent's wife. Tunnel 6 was said by some to be impossible. Nothing like it had ever been done.
It was to be a railroad tunnel linking California with Nevada, over 1700 feet long and 6000 feet altitude, dug through solid granite using black powder, nitroglycerin, and more than 5000 Chinese laborers. Geraldine was to discover many things to terrify. There were explosions, landslides and blizzards. However there were some compensations. She got to run her own telegraph key, without being required to wear corsets.
And then there was that nice, funny looking boy, Stetson Applegate, whom she might, perhaps, fall in love with. And then a third woman showed up, a strange lady named Georgette Dupriest. Though she claimed to be writing a book, no one could quite understand why she was at a railroad camp. Georgette did seem quite taken by that nice gentleman, Mr. Asa Turner. Geraldine begins to have second thoughts when it gradually becomes clear that someone is trying to sabotage the railroad.
And also trying to kill the man she loves. If you read this book, secrets may be revealed.