You'd be surprised how much noise comes through at night, trucks engine braking, loud cars, etc.
I used to live in an apartment on southbank in melbourne that overlooked citylink. Was a pain in the *** trying to get to sleep.
It's probably a noise/desirability factor.
I personally wouldn't consider any house that backed onto a freeway or train tracks.
I think it would be a turn off for potential renters. Perhaps you should go down check it out during rush hour and see what the noise levels are like.