Legend Tours has a page on their website called “Train schedule in Ulaanbaatar (Mongolia)” which contains information about travelling by train from Moscow to Beijing via Mongolia.
One of the paragraphs of useful information for the traveller is the one shown below:
Customs & Immigration. There are major delays of three to six hours at both, the China-Mongolia and Russia-Mongolia borders. Often the trains cross the border during the middle of the night, when the alert Mongolian and Russian officials maintain the upper hand. The whole process is not difficult or a hassle – just annoying because they keep interrupting your sleep. Your passport will be taken for inspection and stamping.
During these stops, you can alight and wander around the station, which is just as well since the toilets on the train are locked during the whole inspection procedure.
This is sort of understatement. Yes, the officialdom part is onerous and a couple of hours each side of the border are given up to much inspecting of documents, checking visas and so on.
In fact, when travelling across from the Mongolian side of the border to the Chinese side, all the Mongolian Immigration officers come through the train in Zamin-uud (and contrary to the article, you are encouraged to remain in the train). Everything is checked, papers and passport. The Mongolian border crossing at Zamin-uud is the only place I have ever been where a customs declaration has to be completed for departure (at the Chinggis Khaan airport in UB, customs forms are only required when arriving). What you should be aware of is that if you travel back INTO Mongolia through Zamin-uud, the Customs folks will want to see the form you completed when you were leaving.
The train then crawls along for maybe 30 minutes or so to cover the 5 kilometres between Zamin-uud and Erlian. The Chinese Immigration folks then take the next 2 hours to check your entry papers. There is a detention area half way between Zamin-uud and Erlian and sometimes the train stops there and a young Chinese guy or two will be escorted off the train and into detention. Presumably their papers are not in order.
Of course, the trip from Beijing to Ulaanbaatar is the same, just the waits are reversed. Seems though that the trains tend to get to the border late at night (when travelling either way). The other thing not mentioned is that Chinese Railways run on Standard Gauge track (4 foot 8 1/2 inches between rails). Mongolian Railways run to Russian Gauge (5 foot between rails). So, at the border, apart from the immigration delays, there is a further delay of a couple of hours while the entire train undergoes a change of bogies. This entails jacking each carriage up and replacing the bogies underneath them. This is done with much bumping and banging whilst the passengers are all still in the train trying to sleep.
As mentioned, the toilets are locked but it is near impossible to get out of the carriage. Also, if it is winter, the temperature in the carriage falls as well. The combination of drinking beer before the border (or coffee) and cold temperature puts an unbelievable strain on one’s plumbing.
Indeed, my friend had saved a couple of plastic beer bottle precisely for this event. Out with the Swiss Army knife, quickly remove the top of the bottle and voila, instant relief.
The one thing that still has me frustrated about this whole process is that there is no reason why the Mongolian AND the Chinese Immigration staff could not check all the passengers at the same time. This would take at least 2 hours off the entire process and reduce the time to travel between Ulaanbaatar and Beijing to about 1 day 4 hours.