Yes, it seems it is now closed - for 2 years!
I've translated the following from the TCDD home page:Due to track works necessary for the Ankara-Istanbul High Speed Train project, the following alterations to service will be in force for 24 months starting from 1st February 2012:
The service of these trains will be suspended:Fatih Ekspresi, Ankara Ekspresi, Anadolu Ekspresi, Meram Ekspresi, Eskişehir Ekspresi, Başkent Ekspresi, Cumhuriyet Ekspresi, Sakarya Ekspresi
and Adapazarı trains
The Boğaziçi Ekspresi
service will run between Ankara and Arifiye (near Adapazarı) only (i.e. it will not run between Arifiye and Haydarpaşa)
The İçanadolu Mavi Treni
will run between Adana and Arifiye (near Adapazarı) only (i.e. it will not run between Arifiye and Haydarpaşa)
The Doğu Ekspresi
which normally runs between Istanbul and Kars will run between Ankara and Kars only;
the Güney Ekspresi
which normally runs between Istanbul and Ankara will run between Ankara and Kurtalan only;
and the Vangölü Ekspresi
which nomally runs between Istanbul and Elazığ will run between Ankara and Elazığ only;i.e. the none of these trains will operate on the section between Ankara and Istanbul.
You can find this information in Turkish on these pages:http://www.tcdd.gov.tr/home/index/