Ulan-Ude to Beijing by train. The Trans Mongolian Railway.
Official Train Operator: www.rzd.ru (English version)
Recent articles covering this route:
‘The Godfather of trains’: the Trans-Mongolian from Moscow to Beijing (Guardian Travel)
Just back from: The Trans-Mongolian Railway (Lonely Planet)
Katie Glass on the Trans-Mongolian Railway (Condé Nast Traveller Magazine)