This isn't a comprehensive packing list, that will depend on who you are, where you're stopping off and what time of year you're travelling. This is merely to give you an idea of what to take on the train, from someone who's currently on board on the trans-Mongolian express!
You can get food in the restaurant cart but, seeing as they're privatised, they are notoriously hit-and-miss! And overpriced! There are often little kiosks at the platforms which are more competitively priced, and at more rural stations women selling homemade food, drinks and cigarettes, but this depends heavily on the time you arrive at the station.
The best way to ensure you're fed and watered is to pack some snacks. Dehydrated food, cuppa soups, noodles etc are great as they're warm and filling and can be made using water from the carriage samovar. Crisps, bread, and individual cheese portions (think dairylea triangles) are also very popular as they don't spoil easily.
There is no cold drinking water, so either bring bottles or a vessel to decant some of the boiling water into to cool. Also, if you drink diet sodas, it might be an idea to stock up on these for the trip, because they are like gold dust in Russia! We found one shop in Moscow that sold diet Pepsi, but other than that it was full fat all the way until we reached Mongolia.
Sweets and chocolate to share with your fellow passengers is a sure fire way to make friends and impart a little bit of luxury.
A bottle of vodka is a good idea whilst in Russian territory, and will make you a popular person!
Collapsible mugs and disposable cutlery are great space savers. Check out sea to summit for their silicone travel mugs etc, they've definitely served us well!
Needless to say there are no showers on the train, and if you're doing the whole trip in one stint that's 6 days without a wash! Wet wipes, anti bacterial hand sanitiser, and tissues are a must have. Toilet paper is not supplied so a large stash of tissues definitely comes in handy. There are mirrors in all of the compartments and in the bathroom, so you can keep up appearances. Dry shampoo is a handy tool, batiste being a personal favourite to keep your hair looking fresh and smelling sweet.
A couple of plastic carrier bags to use as rubbish bags help to keep the compartment clean and tidy, which when sharing a 2x1.5m cabin with three strangers helps keep relations friendly!
Underestimate how much you need to keep you occupied! I boarded the train with a journal to write, coursework to complete, 12 films on my iPad to watch, three books, a travel game and a pack of cards! In reality most of our time has been spent looking out of the window, watching the world go by. The trans-Siberian handbook by Bryan Thomas is definitely high up on my must-have list! Its advice, snippets of information about station stops and km-by-km guide to the journey's sights are invaluable, and have kept our entire carriage entertained and informed for the last week. Playing cards are an international barrier breaker, and are worth bringing for that alone.
Even if you forget to pack a single item, you will enjoy a trip on the trans Siberian as long as you remember to bring a sense of humour and a friendly disposition, and to leave your expectations at home!