This is book 3 of the "Phryne Fisher" series.

This note contains spoilers for the book.

She's almost gassed on a train. Somebody chloroforms the whole first-class carriage. And the missing old woman is found 30 feet from the tracks with no marks for how she got there.

She even uses her pistol to save her life—shooting out the window to clear the chloroform when she's too weak to reach it or break it directly.

So, this is a bit more a technical mystery than many of Miss Fisher's. Nicely done, too. The mountaineer is of course the one who can get on top of the train, get a rope to the watering tower, and so forth.

There's also some white slavery, and Phryne ends up acquiring two teenage daughters.