Transit visa in Beijing for boarding Transmongolia Express

Hi!

My partner and I are planning to fly end of March towards Beijing directly from Hanoi, to board the Transmongolia Express.
Can we use this transit visa for this purpose? Of course we would not be staying longer than the 144hrs allowed, we are Belgian citizens so we are allowed to use it and we would not get of the train before the border with Mongolia and have our onward ticket + Mongolian visa ready.
I just want to make sure if you can us it when exiting China by train instead of by plane?

Besides that I was wondering what is meant by: "Inform the carrier when boarding"? You just need to
inform them at check in that you will apply for a transit visa upon arrival? Will they not make a problem about this, as they are taking a risk by letting you on the plane, as per their point of view?

Many thanks in advance for your help!

Grts,

Katrien