I hope to fly from Beijing to Ulaan Baatar April 25 and return about two weeks later. I planned to fly one way, and book a return train ticket when I reached Mongolia. I have been reading here, however, about the difficulty of getting tickets. Can anyone advise me how difficult it might be to book a one way rail ticket around that time with about a two week advance notice? Thanks for any information.
Did you realize that there are only one or two trains per week from UB to Beijing in early May?
Given this schedule, I would be reluctant to wait to book unless you are willing to take the local train to the border, cross the border and catch another local train (taking an extra day but costing 2/3 the price) if there are no tickets available for the international trains on the day that you want to travel.
Ruth

Three years ago I took the overnight train to the Chinese border. Worth getting first class. You can probably do so with four days notice at the railway station. Taxi across border, probably best then to overnight in Chinese border town and get next Chinese train probably following day. Much more comfortable than overnight bus to Beijing which is what I did.
You probably cannot book international train from Ulaan Baatar.
You can get Chinese visa in U. B. in about a week.
You cannot book the international train from Ulan Bator to either Beijing or Hohhot on the internet. But you can definitely book it through a Mongolia based travel company such as Passport Travel, Selena Travel, Legend Tour or Ocean Travel. The latter was based primarily in Beijing but opened a UB based office last summer. I am pretty sure that the email for all can be found on the internet.
If you decide to go the local train route, it is better to cross the border and spend the night in Erlian waiting for transport to Beijing than to remain in Dzamin Uud. Look for some posts by manulike who described both border towns last summer.
Travel on this route has increased considerably over the last three years. Back then it was easy to get tickets at the last minute. Last year it seemed impossible to get tickets even through an agency months in advance during the high season from late May through mid September.
Ruth

Thank you for the quick and helpful responses. It sounds like I have several options. I did not want to fly both ways, and also did not want to lock myself into a return date before I arrived. I will check out the travel agencies asnd the border town information. Thanks again.