In the UK, a guard with good eyesight and a sprightly, cheerful manner can almost run through the carriages shouting "tickets please!" and see, with a single glance, all s/he needs.
Then she gets to the muppets with phones. They faff, fumble and fuss. They scowl at being interrupted watching videos, browsing social media and listening to music. They can't load their app. They huff and sigh disrespectfully. The guard can't get focus. For a moment they point phones at each other like cowboys in a gunfight standoff. She doesn't have a wifi signal, so now there's a long and embarrassing delay while passenger and guard stare at each others feet.
Finally a joyful beep releases them from technological tension. Everyone in the car sighs with relief and she moves on to the next grinning smombie.
The whole technology is a festering shitshow foisted on people by over-zealous tech peddlers.
Paper tickets rule.