Comment by PetitPrince

I don't think it's fair to conflagrate "Europe" as a single entity. From what I'm reading and my distant recollection:

- In London you top up your Oyster card and pay a fare according to the zone your travelling to. Metro is gated.

- In Paris your load up tickets in your Navigo pass and pay a flat rate every time you take the metro or RER. It's also gated.

- In most (all ?) Swiss cities you don't have any card to charge and have to get a ticket for every travel (like in London your fare amount depends is zone dependent); however there's an app that let you get your location at the start of your travel and end of your travel and automatically charges you the correct fare. There are no gates.