On public transport in Helsinki, the days of buying your bus ticket from the driver are long gone, but there are other ways to get one.

One quite convenient way is through the city transport authority's HSL app, but using it after getting on a bus or tram could be very costly.

App-purchased tickets need to be bought and loaded in the app before boarding, according to the rules. Otherwise being ticketless — even when you're waiting for the ticket to be issued — can still result in a whopping 100-euro fine, if ticket inspectors catch you red-handed.

So, when you jump on a bus at the last second and don't have a valid ticket, be sure to forget the app and use a contactless card reader instead.

Yle asked HSL's ticket inspection unit chief, Satu Koskinen, to explain the difference between waiting to buy a ticket with the card reader rather than on the app.

The general rule is that you should not use the app to buy tickets after boarding.

Koskinen simply explained that rule is part of the app's end-user agreement's terms and conditions.

The rules are stricter for HSL's metro and ferry services, as people need to have valid tickets even before entering the metro platform or seaside dock area.

The rules, however, do have some room for bending, as ticket inspectors are able to use discretion in determining whether to issue the penalty fine to ticketless passengers, according to Koskinen.

Koskinen said that allowing on-board app purchases of tickets would increase the likelihood of people only buying tickets after they notice the inspectors — making it easier for people to travel without bothering to buy one.

The authority's ticket inspectors checked the tickets of more than 3.1 million passengers last year, according to Koskinen.

She noted that the vast majority — more than 96 percent — had valid tickets.

The ticket checks do not always go smoothly, with inspectors sometimes facing threatening situations. But the inspectors' trial use of bodycams has reduced such scenarios, she said.

Ticket inspectors have noted that the mere sight of a bodycam can have a calming effect on potentially fiery exchanges with angry ticketless passengers.

In more extreme situations, matters usually calm down after the inspector informs an unruly passenger that their bodycam is being turned on.

"[That announcement] makes even the most stubborn and opinionated person think a little about their actions when they realise that the incident is actually being recorded," Koskinen said.