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The Trans-Siberian Railway is definitely the longest railway in the world. It starts at Moscow and 9,289 km and seven days later, it finishes in Vladivostok. There are a great many intersecting and parallel railways that are associated with it including the Trans-Manchurian and the Trans-Mongolian, The BAM and Little BAM.
The Trans-Siberian Railway is a miracle of modern engineering through an extremely hostile environment.
Trans-Siberian Handbook by Bryn Thomas.