You'll find that, at some point in the future, China would be very happy to make a more direct connection that involved easier transitions somehow. Well... they've usually beaten a path quite easily before?
China regularly hold talks with Pakistan and relations are good - even apparently China starts to invest in Pakistani ports (and their connections to there). Given :
Where you'd go from Turkey isn't a problem either; Apparently,
Iran and Turkey do have a rail connection.
If China have a good relation with Iran and looking forward to improving connections to the west is among the political talks and Turkey can return to be a "bridge" state (let's admit it's not looking good so far) then theoretically the trains could run to the ends of EU at Greece (or, Turkey), then Turkey, cross through Iran, then Pakistan, and up Kashmir/Tibet towards Chinese east coast.
But even, geopolitics aside, geographically the fact that Earth is round makes the trans-siberian shorter in any case. Yes it involves transshipment but containers are easy to transship anyway (that's what they're for), so going through Russia seems to be the shorter route anyway, and I presume is the case already to a good extent !]
EDIT : Yes Iran and Turkey have one intact rail connection
. Not sure if anything ever pass there though.
EDIT 2 : Yes they did run something there, starts from 2009. More surprisingly it came from Pakistan to Turkey ! Weirdly the turkish end needs a train ferry (why not make something around the lake it has to go through ?), and goly I forgot Pakistan uses indian gauge as well...