Sorry to come over all technical and nerdy now, but given certain atmospheric conditions, it is amazing how far sound will carry. Usually, a very light wind from the desired direction is helpful, but at night with few other sounds to interfere noisy trains ought to be heard from up to 5 miles away.
I have heard Deltics on the ECML climbing Stoke Bank from this distance, and Westerns accelerating away from Reading station from just as far away.
During the day it is not unusual for sound to carry uphill on warm air currents, especially if the sun is shining. I remember being about 1000ft up a hill in the Lake District one hot day and hearing the noises of people in a camp site at the foot of the hill as clear as anything.
But the most amazing sound event (non railway - sadly) I have experienced, was an early morning on Brean Down, just south of Weston Super Mare. There was a light northwesterly wind and from the direction of South Wales I could hear the almost continuous sound of traffic, especially lorries which seemed to be going uphill.
After a while satisfying myself I was not going mad, or misinterpreting sounds from the M5 in almost the opposite direction, I concluded what I could actually hear was the M4 near Newport which must be about 12-15 miles away in a straight line over the Bristol Channel.