The Chinese visa requirements need me to have a ticket out of China.
My plan is to get a dual entry visa for China, fly in to Beijing, visit North Korea by train (I will have this booked in advance), and then take the trans Mongolian express, thus leaving China by land.
Will it suffice if I write a note, stating that I will be leaving China by train? Or do I really need to book this in advance?
Firstly you have to differentiate between getting a visa (decided by the embassy) and being admitted (decided by immigration). Embassies can (and often do) have stricter requirements for obtaining a visa than immigration does for entry.
As for the visa, the requirements vary from embassy to embassy.
The Zurich consulate, for example, told me in person in December that it requires confirmed return/onward tickets of any sort.
Meanwhile, the Washington embassy states on their website that a round-trip air ticket confirmation is required, although I haven't called and asked whether they are flexible about transport modes in practice.
So your only way of knowing is telling the embassy where you plan to apply about your itinerary and asking.
As for getting admitted, the following is stated by Timatic, the database used by airlines:
Travel on a one-way ticket is allowed. Visitors must hold proof of sufficient funds to cover their stay and documents required for their next destination.
The only case where an onward ticket in required is if transiting without a visa, in which case the following two arrangements are applicable at Beijing:
Holders of confirmed onward air, cruise or train tickets for a max. transit time of 24 hours. Transit incl. multiple stops within China (People's Rep.), with a total transit time of max. 24 hours, is permitted. They must travel to a third country
and
TWOV (Transit Without Visa): Nationals of [insert eligible country] holding confirmed onward airline tickets to a third country, if arriving in and departing from the same city: at Beijing (PEK), Guilin (KWL), Harbin (HRB), Kunming (KMG), or Shenyang (SHE) for a max. transit time of 72 hours;
So, in summary, this is what applies at Beijing, notwithstanding any stricter requirements by the embassy:
Observation: Never have we been asked to show a return ticket when entering China. I always have a printout of our tickets with me, never have I pulled it out.
