The Airport authority might have more of an incentive to get T3-D online if:

1) Air Traffic Control can get its act together, and stop being the bottleneck of PEK operations.

2) Enough airlines--including domestic Chinese carriers--don't mind paying the higher price for jetway parking rather than use cheaper remote stands w/bus gates.

I have no clue as to time frame.

I do not know what train you could be talking about for T3-E. There is only the intra-terminal light rail connector that goes from E to (unused)D to C/landside areas of T3. If you meant the main landside part of T3 (-C), yes, there is a separate light rail station area on the Arrivals Level--the Airport Express--that goes into town and connects to the city subway system.