Three main kinds of train accommodation in Russia: Platzkart, Kupe, SV
Why is there a difference in price? Is there any shower, Wi-Fi, kitchen, toilet etc?
Russian Platskart class is an open space carriage, meaning that you share the whole carriage with other passengers. It's an Economy class with 54 people in one carriage with no separate compartments/rooms. However, you have your own bunk bed (upper or lower bed).
Russian Kupe class is a good option if you want to have more privacy. Kupe type of a train car is divided into small rooms (compartments) for 4 people each. There are 9 rooms in a carriage, and in total up to 36 people in one carriage.
The most upgraded class on trains. 2 people in one compartment. Separate rooms that can be locked. There are 9 SV class rooms in a carriage with up to 18 people.
All carriages are slightly different
Usually, there is no shower on trains, but sometimes there could be an exceptions with a shower in a carriage.
There are at least 2 restrooms in every carriage, and there are sinks, too.
Bed linens and towels are provided on trains.
Each carriage of a train has a carriage manager (Provodnik - name in Russian), who checks the passport of each passenger on the train.
Every train has a boiler, so you can make a hot cup of tea.
Usually in the train you can buy snacks, tea, coffee, better take cash for that
Usually there is a restaurant carriage where you can eat by the menu