Some trains in rural areas, especially in cold regions, have doors that can be opened and closed by passengers themselves in order to maintain a comfortable temperature inside the train. On these type of trains, there is an open/close button near the door as shown in the photo. Pressing this button will open the door. If the door does not open automatically, simply push the button.
In some cases, conductorless trains have only one door for getting on and off. If the door doesn't open when the train arrives, don't worry. Simply look around to see if there are any other doors that are open.