There were suggestions recently that people didn't need to buy train tickets before they got on a train because "selling tickets is the conductor's job".
I'll just copy a short paragraph from Rail Staff, Jan/Feb issue, a trade publication for rail employees in the UK; below is a quote:
"
Buy before boarding
A record number of fines were issued to those travelling without a valid ticket in 2021 as TransPennine Express (TPE) [just one of the many UK train operators] reiterated the need for customers to "buy before they board". In total, more than 13,400 penalties were issued to those who chose to travel without a ticket or with an invalid one, resulting in them having to pay either a higher fare or a fixed penalty [a fine].
More than £570,000 was recovered by the train operator through penalties as a result of fare evasion, with 531 people prosecuted at court resulting in both a substantial fine and a criminal record for those found guilty."
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Square brackets [ ] my comments for clarification.
Those taken to court are just the most egregious - most are dealt with on the train or in the station.
So please, dear RS followers, buy your tickets before travel.