The Russian Railways service direct between Paris and Moscow is a classic long-distance train journey. There are actually two legs to the journey, although you remain on the same train throughout. Between Paris and Berlin, the carriages are part of the Deutsche Bahn overnight Paris-Berlin service (450/451). Between Berlin and Moscow they become part of the Moscow Express (13/14).
The sleeping car has ten compartments which are configured as either 3-berths, 2-berths or single berths. You can opt to share a multiple berth compartment with a stranger, if you would rather not pay the extra for a single berth. Each compartment doubles up as a sitting room during the day time, with beds that fold down for the evening. Each room has its own basin, and towels and toiletries are provided.
There is a dining car that serves the train between Rzepin and Warsaw, so make sure you bring some extra food and drink with you to last the journey. The carriage attendant is also able to serve tea to passengers en route.
The journey includes a stop in Berlin of 6hrs (Paris to Moscow), or 10hrs (Moscow to Paris), giving you enough time to do a quick tour of the city. If this route doesn't take your fancy, there are alternative ways of reaching Moscow from Western Europe, including journeys from Cologne, Brussels, Helsinki and even Amsterdam.