Question: How far in advance do you need to book Istanbul-Tehran train or can you just turn up in Istanbul a day or two before the train leaves? Any information much appreciated.
Also what train journeys are inside Iran, what would you recommend?

Answer: It is always suggested to book your train tickets as soon as possible since those are fully sold out far in advance. Using train in Iran is suggested for the long distances as Yazd-Mashad or Tehran to Yazd.

For checking the schedule and train booking follow this link: http://www.uppersia.com/Iran-transport-services/iran-train-ticket-booking-service.html