Eat there, but have other options. The Russian favorite is a styrofoam bowl of dehydrated soup. A hot water dispenser in each car turns it into a quick meal. There're available at kiosks in stations along the way, although it is cheaper to lay in your stock ahead of time. Other culinary possibilies include bread, meats of many descriptions, dried fish, and anything else that can be wrapped in brown paper.
Many stations have groups of peasant women selling various foods, all inexpensive and usually good.
Whatever your food choice, bring enough for others. Eating is a primary diversion and it's fun to make a social event of it. Russians will not take bills that show signs of wear. Only a few places in Moscow and St. Petersburg take credit cards. Traveler's checks don't even make good toilet paper.