Russian Rouble - 1 Rouble = 100 Kopeks
Russian Rouble notes come in the following denominations: 10, 50,100, 500. Coins are also in circulation and come in the following values: 1, 2 and 5 Roubles and 1, 10 and 50 Kopeks.
US Dollars and Euros are the best currency for exchange. You'll find Bureau de Change and ATM machines in Moscow, St Petersburg, Irkutsk and Yekaterinburg. Changing traveller's cheques is cumbersome so avoid if possible and credit cards are rarely accepted outside the main cities.
Russian ATMs allow you to choose to withdraw currency in either Roubles, Euros and US Dollars.
Mongolian Tugrik (MNT) 1 Tugrik = 100 Mongo
The Tugrik is non exportable and money changing facilities are available in Ulaan Baatar. Please make sure you change all remaining currency before leaving Mongolia. The best currency to carry for exchange into Tugrik is the US Dollar. Please note that the USD is sometimes the method of payment for optional activities, for example, horse-riding at Terelj NP.
Chinese Yuan (CNY) 1 Yuan = 10 Jiao
Pound Sterling, US Dollars, Euro and other major currencies can be converted into Chinese Yuan locally, or you can purchase currency in advance. ATM machines and Bureau de Change are available in Beijing.
General Money Guide:
Bring clean, unmarked cash in US Dollars (not $100 bills as they're hard to exchange) for your Trans-Sib adventure.
When changing money, ID is required.
Ensure you have enough in cash aboard the train, or for example, at Listvyanka on Lake Baikal's shore.
Take a mixture of cash and credit/debit cards (for ATM usage). Changing traveller cheques is cumbersome and complicated and only recognised by the very large banks. Avoid if possible.
Credit card acceptance is becoming more common, though cash is still the norm in most establishments.
Check OANDA for the latest exchange rates.