Getting in Beijing by flight and quiting it by train ?

I'm planning traveling Montreal to North Korea for a week with a organized group that will take the the train from Shenyang to North Korea.

I wonder if I can come few days earlier in China by landing at Beijing and taking the train to Shenyang.

Since I'm gonna leave the city of Beijing and leaving the country by another port (Shenyangtrain), I wonder if I'm illegible for the 144 or 72 hours no fees Transit Visa.
Or it is really impossible to me to receive a Transit Visa in a city where I won't take my transit ?