In theory it brings closer the day when it will be possible to travel from London to Beijing via Istanbul by train.
What else needs to be put into place so that I can do this?
First of all, the upgrades on the suburban lines in Istanbul. On the European side:
http://en.wikipedia.org/wiki/İstanbul-Halkalı_Line
On the Asian side:
http://en.wikipedia.org/wiki/Haydarpaşa-Gebze_Line
Currently the Marmaray tunnel itself is running trains under the Bosporus from underground stations on the Asian side to underground stations on the European side. So yes, it is open.
However, the tunnel is supposed to connect these two suburban lines and turn them into a single through-running line -- but currently both of these commuter lines are closed for reconstruction (triple-tracking and speeding up). The Asian line is not supposed to reopen until 2015; the European line might reopen in 2014 or 2015, not sure.
Second, the Istanbul-Ankara high speed line. The Asian-side line is actually closed way past Gebze, all the way to Eskiseher, for construction of the high-speed line. This part is supposed to reopen in 2014.
Third, not strictly necessary but important, is the Bulgarian upgrades. You can currently get to from London to Istanbul by train, but the Bulgarian section is slow and not electrified. By the end of 2015 it should be electrified and good for at least 100 mi/hr.
That gets you high-speed rail from London to Ankara.
Going further east, plans are more speculative. Turkey is extending high-speed rail towards the Iranian border, but slowly; right now there is a train ferry across Lake Van which interrupts the rail line. Turkey will need to build a very expensive rail bypass around Lake Van in order to create a continuous route.
Iran is making various upgrades to its railways from the Turkish to the Pakistani border -- electrification, multi-tracking, grade separation. The Turkey-Iran connection will probably happen; both countries strongly desire it for economic reasons.
Within China, rail has been extended all the way west to Kashgar (the ancient city of Kasia, known to Ptolemey) with passenger service opened in 2011.
Then you get the difficult part -- China to Iran. China is trying to find a route for a standard gauge line (China is interested in bypassing the Russian-controlled Trans-Siberian), but is having trouble.
Pakistan signed an agreement with China in 2009 to study a standard gauge line across the Khunjerab Pass and onward from the Chinese to the Iranian border.... then did absolutely nothing for several years. Pakistan is a mess.
In addition to the Khunjerab Pass proposal, China has previously proposed a standard-gauge route through Kyrgyzstan and this appears to be under investigation.
And according to recent reports, China is currently exploring a Tajikistan-Afghanistan standard gauge route.
As a sad aside, Kushgar is colonized by China. It isn't a Chinese-dialect region, it's Uighur, and the native script is Arabic. China has been demolishing many ancient and historic buildings all over the city, on the grounds of earthquake safety. (The old buildings aren't safe in an earthquake, but y'know, we'd reinforce them.) Of the three great colonial powers in Central Asia, Iran has been the best, followed by Russia, with China being the worst.
