I had a very frustrating experience last summer at Gare du Nord in Paris. I needed RER E (operated by SNCF) tickets for family. Unfortunately, there was a rail strike, so only one of three manned ticket offices was open -- it had a very long line.

The automated machines rejected my USA-issued credit cards, and they did not accept paper currency. I didn't have much in euro coins. Several subway scammers spotted me having difficulty and offered me their "help". Merci, non.

Eventually I figured out a plan that worked: buy a minor item at a store and ask for lots of change in 2-euro coins. I could have saved my family 40 minutes or so if I had started with that plan.