1

Train is now starting in Ankara not in Istanbul, departure every wednesday in Ankara, arrival Friday in Tehran.

2

Dear Sir/madam
Since there is high speed train construction, all trains are cancelled between ankara and istanbul.
So if you want to go to Tahran(Iran) by train you start from Ankara.
You can easily pass from Istanbul to Ankara, there are more than 50 bus companies from Istanbul to Ankara.
Price around 40 TL(1USD=1.78 TL)
Every half an hour you can find at least 4 companies form Istanbul to Ankara.
The departure starts Istanbul-Europa Part from ESENLER MAIN BUS-STATION
The departure starts Istanbul-Asia Part from HAREM ESENLER MAIN BUS STATION.
It takes around 6 hours.

And how do you get ticket and timetable of the train.
You have to visit www.tcdd.gov.tr (Official Turkish Trains web site)

Sincerely

3

As much as I welcome the advice it doesn't answer my original question. Does it make any difference which way you take the train, often you will find that one way is better than the other?

4

Hi, I went there and back my bus ( Istanbul - Tehran - Istanbul), but if you would like to fly one way, just check the prices. It might be cheaper from Istanbul to Tehran and back you would take a train.
The buses go around 50 hours.