Berlin - no turnstiles, controllers get on trains at random and give you a ticket to pay 40 euros within 14 days, after that it goes up, and if you don´t pay it will come up if they ID check you. they might confirm your ID and address. in hundreds of rides it´s only happened to me once though.
Barcelona - turnstiles, many are jumpable, in other stations you can go behind someone. jumping is common enough that there are a fair number of guards watching the entrances, and sometimes ticket checkers in the exits of stations.
Paris - turnstiles, many jumpable, sometimes they check tickets in the station but it´s not that common
Venice - the water buses cost 6€50 each way and the person who anchors at the stop is in theory also the conductor, they don´t ask for tickets every time, but when they did, we just acted confused in english and said we didn´t have any money and they let us off because it seemed unprecedented
Toulouse - high turnstiles on the way in, out, and for transfers, go behind someone
Spain Renfe local trains - ticket checkers sometimes get on (but without a security guard they never check), if you say you don´t have money or documents they usually just kick you off
French regional trains - there is usually a conductor but sometimes you can ride a few stops before they come, if you say you don´t have money or documents you can just get off
French high-speed trains (SNCF) - conductors always walking around, in most cases they will ask for tickets but the interesting thing is that when they are checking they do it in a row in one direction, otherwise when they´re just walking across the train they won´t stop and check. and if they have walked by a few times and you´re sitting calmly, once they´re used to you they might assume you belong there, they pride themselves on never asking the same person twice. when they do ask, if you have ID you can ride to the stop you want with a ticket for a fine that they send to your house but they don´t check the address and even if the address is correct i don´t think anything happens if you don´t pay.
German regional trains: if you don´t have ID they kick you off, if you do, they´ll fine you after checking your address with the police.
Dutch trains: controllers sometimes, they might let you get off, but one time we said we had no documents and they took us to the rail police to identify us
Italian trains: most have controllers that kick you off unless you pay them on the train
Belgium: the metro and trams were no problem, but on a train from Brussels to Antwerp they called the station security, who called the police (I think the police only because my passport was just a photocopy, but in any case they were all extremely nasty and lectured us)
Luxemburg train - we were fined curteously