On most long-distance trains there will be a restaurant car serving freshly cooked local dishes and a variety of hot and cold drinks. Restaurant carts are not open 24 hours a day so keep a note of opening times.
Traditonally, Russian trains have operated on Moscow time, however, journeys from 1st August, which can be purchased from 4th May, will be moving to local time so the restaurant carriage time will be adjusted to local time. You'll be able to find opening hours in the restaurant carriage and can always ask the provodnista for help.
On some trains there will also be snack trolleys wheeled through the train at regularl intervals. If there's no snack trolleys you can often purchase snacks and drinks from the provodnista's cabin.
Each carriage will have a samovar for hot water so you can take tea, coffee, instant noodles and soups. You can pack them in advance of buy them along the way, theyre available at most stations or supermarkets near the stations.
Station shops won't stay open 24/7 so be sure to stock up on supplies during daytime stops.
One of the more interesting places to shop, especially at smaller stops, is with local traders who sell local produce from the train platform or through the windows of each carriage.
It's also best to pack your own cutlery and travel mug.