Complex sentence: At Giurgiu, passengers were ferried across the Danube to Ruse, Bulgaria to pick up another train to Varna, from where they completed their journey to Istanbul (then called "Constantinople") by ferry.
Split into simpler sentences: At Giurgiu, passengers were ferried across the Danube to Ruse, Bulgaria to pick up another train to Varna. They then completed their journey to Istanbul (then called "Constantinople") by ferry.