Help with trains in Iran (Tehran)

Hello,
I will visit Iran in August and I was told that in order to secure a place in a train it is necessary to book tickets several weeks in advance. I would like to know if it is really like that because I have no way to buy tickets while in my country. I'm planning to use trains from Tehran to Esfahan, Esfahan to Shiraz and Shiraz to Tehran.
Also, I cannot find on the net timetables. Anyone knows where I can find this?

Thanks!
sandra