Random thoughts:

• When packing some food for the trip I really recommend going to a big store and getting things you know well.
• I recommend to avoid kvass if you didn't drink it a lot before. Same thing for kefir and its relatives. They can be a digestion problem for those who are not accustomed to them. Any fermented food is not a good option during travel and we, Russians, have a lot of fermented food in our cuisine.
• There is always someone selling food on train (a bar or something) but the longer the route the more hilarious the prices.
• Check out those for sweet snacks. They never cause problems. Throw away the outer cardbox after buying because it is 3 times bigger than the package inside.
• If you plan eating in cafe or restaurant, have your own knife. They have knifes more blunt than spoons usually.
• The announcer on the railway station warns against thieves who share poisoned food with passengers on the train and browse their things while people are unconscious.
• Not a food advise, but: buy a VERY high-quality charger for your gadgets. Onboard electrical outlets have very low quality of power, this really fries up cheap chargers. Have a spare one. Turn your phone to plane mode when out of cell tower range - it really saves battery.
• In case it isn't clear: babooshkas selling food on the small train stations not only have no certifications on their produce, but don't even suspect such documents exist. Same is true for the shady kiosks with the size of a washing machine.

Eating utensils

A mug is a must. Also, a spoon, a fork and a knife might be really useful. A bowl is also good to have if you have enough space. Of course, make sure these are from plastic so that they will not break if fall down to floor, and make sure they are suitable for hot water.

Paper napkins is also good to have.

Where to shop

Avoid shopping directly at the rail station unless you don't have other option (such as if it is an intermediate station and you have just half an hour while you train stops there). Better find a decent supermarket. At Yekaterinburg there seems to be a ?????? (Magnet) supermarket not far from the rail station, so better shop there.

Soft drinks

Some drinking water is definitely a must. It is very common to be rather hot in a train (either during summer during hot weather, or during winter if the train attendant switches the heating on), and the only source for room-temperature drinking water will be the train attendant selling small bottles; but they might soon run out of stock. So buy enough drinking water in advance. They are usually sold in 1.5-liter bottles; if you do not have any preference, then just buy the cheapest. They can be carbonated (????????????) or uncarbonated (??????????????), choose what you like. Buy at least one bottle, or, depending on your strength and the available place in your bags, buy up to 3 bottles. Make sure you replenish your supply during the trip at intermediate stations.

If you like hot tea or instant coffee, buy some tea bags or instant coffee packs; hot water will be available in the carriage. You will also be able to get those from train attendant, but they will be overpriced and the selection will be very small.

You might also buy other soft drinks (cola or juices, etc.) instead of water, but I still suggest you having at least one bottle of just water.

Meat etc

You will not need much calories on train, as you will mostly just sit or sleep, but still take something substantial nourishing food. If you eat meat, take some pre-cooked meat. The most common choice is chicken; you will most probably be able to buy boiled or smoked chicken either at big supermarkets, either at small kiosks on street or at a rail station. You can also buy pre-coocked meat in special sections in a supermarket.

Another option is smoked pork sold on street stands called "shaurma" (??????), "donner" (??????) or "kebab" (?????). Most Russians frown at it, saying it is made from stray dogs and cats, and I have never tried it, but I have seen several reviews from foreigners saying that this is the best smoked meat they have ever eaten.

If you are staying in a hostel or apartments, you can also boil several eggs and take them on train.

Pizza from a lot of pizzerias is also a good option, although it will smell a bit.

Have also a bit of salt and sugar; sugar is in general good for tea/coffee/dairy products too.

Pastries

You can buy a lot of small pies from any grocery store. Also, if you are taking some meat or eggs, you might buy also a loaf of common bread.

Some people like to take some chips, cookies, etc.

Dairy products

Russia has a rich culture of different dairy products not commonly seen in Europe or in US, and you definitely should try it. Some of them are also good to take with you on train. Kefir (?????), ryazhenka (???????), even smetana (???????) are good, just note that they should normally be kept in a fridge, so make sure you eat them on the first day of your trip. You may want to add sugar to them. Yoghurt is another option, and it may last several days. Simple milk is also good if you like it. For all dairy products, make sure the packaging is strong enough.

Instant food

You can buy instant pasta, instant soup or instant smashed potatoes -- you will just need to add boiling water to them. Personally I do not like it, but many people prefer this food on trains.

Alcohol

I absolutely strongly advise against taking or drinking anything stronger than beer. Do not take vodka with you, and do not join strangers in drinking vodka. There are stories of criminals who will offer you vodka (and you are lucky if it is just vodka without any narcotics added), and then rob you while you are unconscious. As a foreigner, you will be an obvious target for them. Moreover, even non-criminals can become quite unreasonable when drunk, and I don't think you want to fight with some drunken men.

Beer is more ok, although I will still advise against it. Better have your head sober, and drink alcohol in safer places.

Also note that attitude to alcohol will depend on the type of carriage. In 3rd class (platskart) almost half of the carriage will be drinking beer, some even vodka; in cheap 2nd class (kupe) you still have a good chance to have your neigbours drinking; but in more expensive kupe you will probably not meet drinking people.

Other

Make sure you have some medicine in case you will have problem with your stomach. This is a general advice for any trip, but for train it is especially important.

Have chargers for your mobile phone, ets. There are 220V sockets in the carriages: usually near the toilets, and sometimes somewhere inside. It is also good to have extension cord with several sockets, so that you will not have to wait until the wall socket is available. I.e. if you see another passenger using a socket, and you need it too, you can just offer to use your extension cord.

In addition, consider to take some Doshirak lunchboxes (it's usual for Russians), fast oatmeals, buns or cakes and at least 2 bottles of still water (not lemonade!). It's also OK to take apples or other fruits, except of oranges (oranges are not the best choice for travelers), cucumbers.

Drink tea for saving your own water.

Be sure you have plenty of tissues.

There is hot water in each carriage, so do for drinking instant coffee or tea is a good choice. If you want to "eat cheap", instant noodles / instant food is the right choice for you.

Otherwise, the food in the dining car is also very good, you will get typical Russian food, and also it is not that expensive (usually US$5 - 8 in Rouble).