explicit law

This "penalty fare" law requires an application to government for each station to be covered, and extensive signage and reminder messages at every entrance, the provision of working ticket machines or 24 hour ticket staff with a monitoring system to ensure the law isn't applied if the machines break or the staff fall ill, and so on.

Previously (and still today where the train operator hasn't put all this in place and made a formal application to use the law) the only thing the inspectors could do was demand that someone travelling without a ticket bought one there and then. If the person refused, their only recourse was to try to take them to court (in most cases the individual would refuse to identify themselves and flee the scene) and even if it got to court the case was likely to fail if the defendant offered to pay the ticket price and said they'd forgotten to buy a ticket or had some other even halfway plausible excuse.

So essentially until this law was put on the books, the train operators had to rely on their passengers being either honest (and buying a ticket voluntarily) or stupid. Fortunately a lot of people are honest.