What meals are available on Russian trains?

This is the dining car on the train between Perm and Moscow (Trans Siberian)...

As shown, there are ample provisions with the most concern devoted to alcoholic beverages. But there's nearly every kind of food available for snacking and hot meals. The dining cars are well above UK standard and minimally equal to or superior to European standard.

You can get to the dining car from 3rd class.

On your specific question, about the smoked fish (presumably smoked Omul) from Lake Baikal... You would certainly be able to get it on trains originating in Irkutsk (???????) and possibly trains on the route from Perm (?????) heading eastward (namely the Transsiberian).

But I doubt that smoked fish from Lake Baikal would be available on other legs. There's also some seasonal variations because lots of the fishing villages on the lake are not accessible by road (plus ice on the lake is up to two meters thick in winter). I would be surprised to see it on trains serving western Russia. What this amalgamates to is my suggestion to 'stock up' in Irkutsk.

Failing train availability, you would be able to get smoked Omul from Lake Baikal at the food section of GUM (???) in Moscow (assuming you are well-minted).

For your other question about hot water, this is covered by regulation backed by long-standing convention. No problem.

Here is a (blurry, sorry!) picture of the menu as I found it when travelling on the Trans-Siberian Railroad in 2013.

Here are some further pictures of the specialties, pancakes, and appetizers.

They have quite a respectable assortment, and I think they actually had most of it in stock (in contrast to the Chinese restaurant car, which was out of nearly everything and was closed most of the time). By Western standards the prices are quite reasonable (the salmon, for instance, is the equivalent of €6,50) and the quality is quite good. Almost everything seems to be prepared with dill and vinegar, though. We found the restaurant car empty most of the time, and we were usually the only customers.

You can also buy food from ladies on the platform, and since there are only a few stops per day, if it does stop, it stops for more than enough time to let you do business. The selection is very varied, ranging from fresh vegetabeles, snacks and candy bars, to smoked fish and savory pastries.

More pictures.

And yes, they do have smoked fish. I'm not sure if this particular one is Omul from Lake Baikal, but it certainly did stink up the carriage like proper Omul would.

Hot water (and they mean business, it's boiling!) is always available from your carriage's samovar:

(Actual working Samovar, not a museum piece)

It's a very good idea to bring some instant coffee, noodles or freeze-dried camping meals and cups and bowls to prepare said items. Anything you can add boiling water to works.

In addition to other answer: Every long-distance train contain a car with restaurant in it, it's located in a middle of the train, and it works nearly 24-long, so you can try out the food there. Also there are always some snacks you can buy on the car-holder. As other's mentioned, on some stations you can find a local food, but be cautious with it as there is no control on them from medical and hygienic services.

Am I right you are saying about Couchette class as the 3d class? If so, there always be a water cooler or boiler with hot water for you. It's for free, and must be all the way long your trip.

My information is a bit outdated (1999 and 2000), but you can expect every carriage on long distance trains to have a samovar with boiling hot water available to you.

As @michael-borgwardt points out, at plenty of stops outsiders will be selling food. And they will do so on the stops where they know there will be enough time for you to buy it.

I also seem to recall that, on some trains, food is available on the trains, as well. But, my memory is a bit hazy on that.

At nearly every stop, there are local people selling food to the travellers.

Source: TV documentaries I've seen.

I used to live in Russia. Yes, there is usually a restaurant car in the middle of long haul train. Yes, the water boilers is available in every single car, so you can get tea or coffee. But that is it. Answering your question, you should not expect any quality of food on the train or on any stops. Try to avoid it, you may then get a dissent vacation. If u stop over in some city/villages, ask locals, usually you can find some local shops with good quality of smoked fish. There is probably a drinking water on the train or you can buy it during stops, but common get 3-5 bottles before boarding to avoid surprises, especially if you are traveling with kids.