Different stories on the origin of the temple.
First one goes like this. Once upon a time the Vindhya Paravat requested Lord Shiva to take abode on its fold; the Lord agreed and came to Omkareshwar.
Second one tells us that two Kings of Mandhata dynasty, King Ambarish and King Mandhata, requested Lord Shiva to take abode at their country. The Lord agreed and came to Omkareshwar.
Third story is that Lord Shiva came to the earth (at Omkareshwar) to kill the demons on request from devas.
The temple is on the banks of river Narmada on an island formed by river Narmada. The island is said to be in the form of ‘OM’ (the pranav mantra) hence the name Omkareshwar.
The temple is on a remote place; one can only wonder how people reached this place even a few years back when the means of transport is much less than now!!
Nearest major city is Indore, which is 77 kilometers from here.
The nearest rail head is Omkareshwar road that is 12 kilometers from the temple. But, it is a small meter-gauge station; not many trains are there.
So it is safe to consider either Indore or Khandwa (on the Delhi- Mumbai main line, 140 kilometers from Omakareshwar) as the convenient rail head.