As can be seen from the quote below, it is not easy in many ways for North Koreans to buy tickets at train stations.

North Korea does not run long-distance buses. There is a long line at the station to buy tickets. The number of sales per day is fixed, and it is not always possible to buy them even if they are lined up early. Good people give bribes to station staff to buy tickets. (Toshio Miyazuka, “Wandering Children and Beauty Corps: Living in North Korea”)
You cannot buy a railway ticket without a specially issued pass. (Alexander Zhebin, The Kim Dynasty I See)
Autumn of 1993. Already by this time, trains heading north from Pyongyang were sparse, if at all, due to lack of power. Even if a ticket could be bought, the chances of sitting were small, unless the traveler was a senior official of the party.
The station was always full of passengers waiting for the train. They roamed the dark camp, squatted down, smoked, and waited for the train to arrive. When the train arrived, he rushed wildly, pushing his body through a broken window and climbing the seam of the vehicle. (Barbara Demick, Nothing to Envy:Ordinary Lives in North Korea)

He is arrested without a certificate or ticket and is forced into forced labor.

If a person who travels without a certificate or ticket is caught, there is a detention house that will force them to work for one or two months without bribes and six months for barefoot. A major representative is the “management detention house” that has been arrested and detained for entering or trying to enter Pyongyang city. There are no small or large “forced labor camps” near the five stations. (Jang Ki-hong, “North Korea: ordinary people”)