Plane and Train: Beijing to Hong Kong

I am interested in flying into BJ from Russia. As an American, I know if I have a booked flight for Hong Kong from BJ, I can receive the 144 hour transit waiver.
However, is it possible to fly into BJ and the take the overnight train to Hong Kong? The train leaves from BJ West so it seems like it should be possible, but I want to confirm that first. Thanks!