Could be very cold. But then again, it´ll probably depend on the year, so there may not be a definite answer until the time comes.
Went from Europe to India via Turkey & Iran in Oct/Nov 2007, and mostly the weather was pleasant. But the areas around Lake Van, Dogubayazit & Bazargan border were getting chilly. Because of altitude, that area has a severe winter, that lasts for many months.
There was someone stuck in Turkey waiting for visas at that time, and I believe he was also researching to put his bike on a train to Tehran... I dont know more about this, but it could be an option.