Dreadnought

Mercy Lynch is working at a war hospital in Richmond, Virginia, when she learns that her husband has died in a POW camp and her estranged father is gravely injured and wishes to see her. With no good reason to stay in Virginia, Mercy sets out to see her father in Seattle.
But crossing the country is no small task; it's a harrowing adventure through war-torn border states by dirigible, rail, and the Mississippi River. And once Mercy finally arrives in St. Louis, the only Tacoma-bound train is pulled by a terrifying Union-operated steam engine called the Dreadnought. Lacking options and running out of money, Mercy buys a ticket and climbs aboard.
What ought to be a quiet trip turns deadly when the train is beset by bushwackers, then vigorously attacked by a band of Rebel soldiers. The train is moving away from battle lines into the vast, unincorporated west, so Mercy can't imagine why it's meeting such resisitance. Perhaps it has something to do with the mysterious cargo in the second and last train cars?
Mercy is just a frustrated nurse who wants to see her father before he dies. But she'll have to survive both Union intrigue and Confederate opposition if she wants to make it off the Dreadnought alive.