Changing money.
On the train from Beijing, once over the border the restaurant car will have been changed from the Chinese one for a Mongolian one. If you're lucky, they may still accept smaller Chinese yuan notes, but if you go in there with no change, they'll offer to change it for Mongolian tugruk. DON'T accept - the rate will be approximately half what it should be; for example I only had a 100yuan note, and he told me the rate was 100T to 1Y, so offered 10000T in exchange. Luckily I'd already checked the exchange rates and knew that it should be closer to 200T to the yuan, and told him so! He wouldn't give me the correct rate, and I wasn't changing at his!









