Trans-Siberian Express

It is one of the greatest, and almost certainly the longest single train journey on earth: the Trans-Siberian.
In reality the Trans-Siberian is three different trains on three different routes. The proper Trans-Siberian train (the Rossiya) leaves Moscow every other day for the 9258 km, seven day, journey to Vladivostok on the Pacific coast near Japan. The problem with this train is you have to either begin or end in Vladivostok, which is not really on the tourist map. The whole journey costs between $500 and $850.
The slightly more popular route takes you to Beijing China. The Trans-Manchurian (train #20) leaves Moscow once a week, and costs between $500 and $700.
For touists the most popular route, and the best journey between Moscow and Beijing, runs through Mongolia. The Trans-Mongolian (train #3/4), leaving Moscow on Tuesdays, is a 7621 km, 6 day journey which crosses the Gobi desert and affords spectacular views. The trip costs between $450 to $700.
VISAS
Yes, you need a visa or visas for these great trips. The Trans-Siberian trip stays inside Russia, so a Russian visa is all you require.
The trans-Manchuria train rides the rails in both Russia and China, and you will need both Russian and Chinese visas.
The Trans-Mogolian train takes you through Russia, China and Mongolia, and all three countries require visas.
For those of you who like trivia, the true longest single train journey on earth is a Russian train which runs from Donetsk, Ukraine, through Mowcow towards Vladivostok, then on to Pyongyang in North Korea.