I see this once a week. I saw it again yesterday, mother and daughter. They put up a heck of a fight. The ticket inspector has no choice. His job description is no leniency ... PERIOD. When I see it before the consequences fall upon the tourist, I prevent it. But once the ticket inspector checks and sees your ticket was not validated, then there absolutely nothing you can say or so. You can be Mother Teresa, and you will still pay. I hope you have your passport on you because they will want that number if you dont pay on the spot. If you pay the fine on the spot or within two days at a BKK office the fine is 12,000 ft ($36) for each unvalidated ticket in your group or 25,000 ft within 30 days or 50,000 ft there after. Oh, they will take a credit card. Understand thats a $36 fine for not validating a ticket that cost $1.25.
The metros have electronic validation machines at the entrances to the stations. Hard to miss, easy to use.
The buses and trams and trolleys and commuter trains have validation machines inside, near the doors. Mostly electric except on some of the old trams and commuter trains. Those can be manual. Manual means put the ticket in and pull down on the slot the ticket is in. https://youtu.be/Db5dLQS-YpE?si=EQ3K9U3zXTrXQu39
How to avoid all of this? Buy a travelcard for the duration of your stay. No validation required.
24 hour: https://bkk.hu/en/tickets-and-passes/prices/24-hour-budapest-travelcard/
72 hour: https://bkk.hu/en/tickets-and-passes/prices/72-hour-budapest-travelcard/
If you are staying more than 72 hours (smart move), then get (it costs about 75cents more than the 72 hour card)
15 day: https://bkk.hu/en/tickets-and-passes/prices/15-day-budapest-pass/
Paper or electronic. In my mind the paper is a lot more convenient than messing with your phone all of the time. But either is fine. The locals like cell phone cases with clear backs and they put the paper ticket under the clear back. When I had a wife and three kids to keep track of, I made them use these: https://m.media-amazon.com/images/I/71dnWRRztIL.jpg
I understand that this does not really pertain to RS Travelers (footnote 1), but thought it might be helpful for your friends that might come to Budapest.
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1. Travelers, Tourists, and all Citizens regardless of country of citizenship who are over the age of 65 travel for free on all public transportation, including trains within Hungary.