A long and heavily laden passenger-train-the 3 o'clock limited express from Boston to New York-and composed chiefly of parlor-cars, was almost ready to pull out of the station. The engineer and fireman were in their places, while the porters, standing beside their steps, were awaiting the last signal from the gong. Midway of the train, and sitting at the open window of her section, a young girl of perhaps fourteen or fifteen years, was sitting. She...