I took the train number 5 (UB to Moscow) in late February. There was no restaurant car! There were also no people on the platforms selling food, so make sure you stock up at a supermarket in UB or at the train station before boarding.
The visa form at the Mongolia-Russia border crossing did not have any English. Fortunately, a Mongolian passenger who spoke English helped me fill out the form. The Russian visa officer seemed rather disappointed.
There was a lot of shifting of luggage over this border crossing, and a number of people trying to store things in my cabin. The Russians do a thorough search of each cabin, so don't lie on your form!