Comment by mailund

In Norway, the trains and subway don't have physical barriers, you just enter! You will get a fine if you are stopped in a random control without a valid ticket (although the controllers are apparently reasonably lenient if you can show that you usually have an active ticket and just happened to forget), but there's nothing physically stopping you from trying!

I believe the reason is mostly historical (it wasn't deemed necessary when most of the system was built, and the later attempt to introduce the gates later failed for a bunch of different reasons)