The River Horse Tsar. The Thrilling Adventures of the Most Dangerous Woman in Europe, #6

A desperate father hides a little slip of solid gold in the back of an artist's canvas. It is a token, the clue to a vast international conspiracy to assassinate a world leader and change the course of the 19th century. A train crash puts it into the hands of Marian Halcombe Camlet and Walter Hartright. They unravel its secrets in a breakneck chase that takes them across Europe, through kidnappings, catacombs and nunneries, a major project to domesticate the hippopotamus for the British Army and, at the last, deep into the depths of the cruel dilemmas of women.
Even the most dangerous woman in Europe can do only so much, when every law in Victoria's Britain is weighted in favor of men.