Otherwise, the dining car serves salads, soups, and dishes based on chicken or beef. It is good food. Eat there, but it may be a good idea to bring along some of your own food and snacks. There is a hot water dispenser in each car, so anytime you can make an instant soup, or coffee, or tea. There're snack kiosks in stations along the way, too, where you can buy fresh breads, meats of many descriptions, dried fish, and fruits and vegetables.
Many stations have groups of peasant women selling various foods, all inexpensive and very tasty.
Eating is a primary diversion, and it is a Russian tradition to make a social event of it. Russians will not take bills that show signs of wear. In most in Moscow and St. Petersburg, and in othert major cities on your route they take credit cards - though not on your train. Don't bother with Traveler's checks, you will have hard time trying to turn them into money. Have singles and fives handy, you will find it quite useful.