> I don't think I have been checked a single time in the last 2 years.
My experience differs a lot: I go by train (in Germany) rather often and would say that it was more common to be checked than not (i.e. for me the probability of being checked was more than 1/2).
like public "city" transportation or regional trains?
in regional trains you get checked more often (longer distances, less passenger exchange and per-train personal have this as a side effect), but already it's somewhat tricky to have a regional train ticket which is not linked to your name
in public "city" transportation it's semi-random but also there are times & stations where statistically it's most likely to find someone without an ticket and in turn this places/times get checked very frequent and if you are there every day at the specific time-frames you are unlucky and also get checked very frequent. But most times and place combinations do not have this issue and some are close to guaranteed that you don't get checked.
My experience differs a lot: I go by train (in Germany) rather often and would say that it was more common to be checked than not (i.e. for me the probability of being checked was more than 1/2).