Passenger train types
Though every country can have it's own passenger train classification there are some principles common to all the countries of the former USSR. There're two main train cathegories: passenger and suburban.
Usually all the trains with numbered seats where passenger gets its place are called passenger. These trains are usually locomotive-hauled. The passenger needs to buy a ticket for specific train for specific date. (Not earlier than 45 days before train departure.)
It's allowed to return passenger train ticket to ticket-office and to get money back (not all the money though).
Automated computer system called "Express-3"e; is used for ticket booking and selling, so it's possible to buy a train ticket in any country of exUSSR. But it's important to note that if you buy train ticket to take a train within the other country (no border crossing) it would be significantly more expensive than buying this ticked in the country you are going to travel in.
All passenger train types have common transportation rules, but these rules depend on what countries you're travelling too. For example to get the ticket to Europe (outside ex USSR countries) you need to buy it in specail ticket-office called "international". But you don't need internation ticket-office to buy a ticket inside CIS, Latvia, Lithuania, Estonia, Georgia. Passenger passport maybe needed to buy a ticket.
Passenger trains can be local, long-distance (quick or passenger) and hi-speed.
Suburban trains have "unlimited" number of seats.
A passenger buys ticket from station A to station B and can take any suburban train a day. For this reason it's not allowed to return suburban tickets to the ticket office (otherwise it could be possibe to ride a train and to get money back).
Electric multiple-unit and diesel multiple-unit trains are usually used for suburban traffic.
Suburban train routes are usually not longer than 150 km, but in several cases the train can change its number and to be considered to go a new route up to 150 km. A train can change it's number several times.
Sometimes passenger need to change suburban trains, usually he can buy just one ticket for all route long.
Suburban railway station may not have ticket office so passengers need to buy a ticket from a conductor.
Some railways operate accelerated suburban trains. They are usually more comfortable and can have numbered seats (in this case the other kind of ticket is needed).