On the Z trains, I used to enjoy hanging out the dining cars because some of them were uniquely decorated and the atmosphere was just plain interesting. However, the D train dining cars (at least the ones I've seen) are more reminiscent of the cafeterias in local office buildings---> I don't go there, but if I was really in the mood to meet other people, that would be a logical choice.

Apart from that, I often end up in a room with 3 other strangers, who invariably want to talk to me. I usually play along for a bit before moving to the stool in the aisle outside of the room with my computer and/or reading material.

I rarely sleep before midnight anyway, but even if I did, it would have been difficult staying asleep with near constant barrage of (really loud) announcements (in Chinese and English) during the first 2.5 hours... "We will be arriving at Wuxi in 5 minutes", "We are arriving at Wuxi Stn now", "welcome aboard our train to South of Beijing Station. The next stop will be Nanjing West", etc, etc.