I got a fairly harsh reminder last night that Serbia is not part of the European Union. I was travelling on a night train from Budapest to Belgrade and there were six of us in the compartment, including a Greek couple who were going back home from Vienna. On the way out they had flown so no problem there. On the way back, they missed their flight and decided to travel by train rather than pay for a new plane ticket. To do this they needed to physically travel through Serbia and Macedonia. When Serbian border guards came on board they asked for passports. The guy had his but the girl only had a national ID card, as it is enough to travel all through the European Union. Unfortunately, that didn't cut it with the Serbian officials and they were asked off the train. I hope they found a way to get home and are not stuck in the middle of nowhere. So, when travelling to the Balkans, take your passport with you.