If your suitcase is left on a train, you should as soon as possible contact the carrier. Most of them provide relevant telephone numbers on their websites, so that you can contact them with regard to the lost luggage. Updated list of these contact numbers can also be found on UTK's website (Polish version only).
You can use regular carrier information channels as well (the numbers are provided on the railway schedules, placed on station platforms). If the train is equipped with video monitoring, you should additionally ask for it to be secured – in case you suitcase "falls into the wrong hands”.
According to regulations, depending on where your luggage was placed on a train, either you are or the carrier is responsible for it being lost. If you place your luggage within sight – f.e. on the luggage shelf in the compartment or in the seat next to you – the carrier only bears responsibility if the damage/loss was caused by the carrier. It is you who is responsible for the luggage placed that way.
However the case is quite different when a passenger him-/herself is not able to take care of his/her luggage and places it in the storage area designed for such purpose. This means f.e. luggage shelves in non-compartment wagons. Frequently such shelves are placed in a given distance from the passenger’s seat and not visible for him/her. In such case it is the carrier that bears responsibility for the lost luggage, no matter whether it was the carrier’s fault or not. If, during travel, such luggage is lost, the carrier will have to reimburse to cost of it. In such instance you may have to document your loss. A similar responsibility is born when it comes to travelling in a sleeping car.
However, the carrier bears no responsibility with regard to loss of: money, securities as well as valuable items, being of scientific or artistic value. There’s an exception to this clause, where the carrier bears responsibility for such damage or loss, when it occurs due to the carrier’s intentional fault or gross neglect of duty.