Don provided a very thorough overview of what to consider. (TU) It is my hope you choose what is best for the homeowner (their timeline may be the most significant factor) rather than what is most expedient for the listing agent.
If this home possesses 'one of a kind' attributes (noise is not an attribute) that adds to quality of life or even tangible value, your how owner may be better served by waiting for 'the' buyer. I've sold noise homes to folks who never go outside and to those whose outside venture was a plunge in the pool and back inside they went. Some didn't even hear the background noise because of the environment they just left.
The level of the noise is a significant factor. If one can continue a conversation the problem is minor. If however, one must signal a 'time-out' for the train to pass, that would not be minor.
Please, as a last resort, sacrifice the homeowners equity and lower the price.... more