Tom, in Germany, for example, you have to approach the conductor as soon as possible, preferably before the journey begins. Otherwise you will be assumed to be riding without a ticket when he comes to your seat. "The machine was broken" is not an acceptable excuse, I have read. (Many small German stations are completely unattended.)
I think the OP, and some other posters are not giving enough weight to the local practice of an "honor system" that includes weekly and monthly "subscription" tickets. Now, I'm mushing up in-city busses and trains with long-distance trains, but the assumption is the same: Supervision is lax, because the assumption is that everyone has paid (or validated their ticket, or whatever) before they board. It's important to consider that Europeans expect mass transit to be subsidized by the government. Americans with Chevy Suburbans and Lincoln Navigators won't vote for politicians who subsidize mass transit!
There's a long tradition that "daily" railpasses have to be signed and dated before the train starts up. Otherwise you are assumed to be hoping that the conductor will not get around to checking your ticket. The same is true of 10-journey open tickets I've bought in Belgium.
In a desperate attempt to reduce labor costs, any labor costs, cash is not accepted on some busses in the United States, or change is not made. It may be framed as brilliant management to avoid a fare increase this year, but it all comes down to taxpayer reluctance to pay for government services, even ones they expect and use. I've heard people telling others that they should include bridge tolls in their sum of monthly "taxes". But a bridge toll is a "user fee", and we certainly know that bridges fall into the river if they are not painted and repaired.
As someone who started riding the NY Subway in the 1950's, I was astonished when the Transit Authority started closing token booths (in the 2000's) but left conventional "low" turnstiles in completely unstaffed entrances. They had previously prioritized catching and prosecuting fare evasion, in a zealous, Puritan way. They used to put high-turnstiles (also called "iron maidens") in unstaffed locations to prevent evasion. But the desire to save money finally took priority over everything! And the police occasionally lie in wait for turnstile-vaulters. And of course you lose the safety function of a live token booth clerk.