The original Trans-Siberian Railway connects Moscow and Vladivostok, covers 9,829 km and crosses eight time zones, and is the longest railway in the world. It takes seven days to travel from Moscow to Vladivostok without stopping. The mainline of Ulan-Ude branches in three directions. The Trans-Siberian part stretches to Vladivostok, the Trans-Manchurian line runs through northern China to Beijing, and the Trans-Mongolian line also ends in the Chinese capital but passes through Ulan Bator.
If the goal of the trip is to cross the maximum distance by rail in the shortest possible time and you prefer to enjoy nature from the window and communicate with the other passengers, so all you need is to buy a ticket for a direct train on the Trans-Siberian, Trans-Mongolian or Trans-Manchurian route. The original Transsiberian journey will take 7 days, the Trans-Manchurian journey will take 6 days, and the train journey to Ulan-Bator will take about 5 days.
The most famous direct train from Moscow to Vladivostok is "Rossiya" # 001/002. Until 2020, it was not only the most comfortable but also the fastest train on the route. Now it changes the schedule by adding more stops on the route, so it is not the fastest but still comfortable train. The carriage classes are Kupe (second class quadruple sleeping compartments) and Platskart (dormitory-style sleeping carriages with open units). Alternatively, there is train # 061/062 on the same route, but with fewer stops so a little bit faster, and there are also first-class carriages (sleeping compartments with two beds). Also, many other regular trains run on the shortest part of the Trans-Siberian route, so you can combine several trains according to your itinerary and preferences. Electronic tickets are available for all these trains running in Russia, which means you can get tickets online.
Although the main Trans-Siberian line runs from Moscow to Vladivostok, most tourists go to China in one of two branches - Transmongolian or Transmanchurian. There are two direct trains a week connecting Moscow and Beijing, trains 3/4 through Mongolia using Chinese buses, and trains 19/20 Vostok through Manchuria using Russian buses. There is also a Mongolian train No. 1 between Moscow and Ulan-Baator. 5/6. You can either book a direct trip on one of these trains, or arrange a stop service that connects several trains.