Here's the info I found - Varna was part of the very first variation of the Orient Express line. Due to WW and other factors the line changed during the years. The 2 original endpoints are Paris and Istanbul. In between the first train departed on October 4, 1883. Starting form Paris, via Munich, Vienna, to Romania. At Giurgiu, passengers were ferried across the Dannube to Ruse and catch the Ruse-Varna train. From there to Istanbul passenger traveled by ferry. Some sources say the time necessary to do the trip was 6 days.
I can only imagine what an adventure it was! May be one day it may become a cultural route including ferry form Varna to Istanbul.
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Varna railway station
You definitely should check this article - it shows nowadays photos of the Orient Express journey.
PS: Many thanks to Irina Atanasova for this hint :)
