Railway work hum across the fields in the small hours

Things happen in the night that we know little of. Streaming sometimes reveals them. I like to leave it playing at a low level while I sleep. Maybe this effects my dreams, but sometimes I'm awoken; on this occasion by a fairly loud rather musical hum. It sounded fairly close even though I knew the mics were some distance out in the fields. I got up, opened a window and was surprised to hear the same hum just outside. Intrigued I got dressed and went to explore. It turned out be work on the railway, the droning machine engaged in some heavy repairs in the small hours while trains were stopped. It's sound pervaded the whole landscape, heard by me in town and by the mics in the hedge 1.7km away. As the work moved slowly down the track the town became quiet again, but it remained audible in the fields for considerably longer. A good lesson in acoustic geography and an illustration of the sonic lay lines propagated by air currents and channeled by contours through the surrounding land.