Thank You. I am a music teacher and I like to give references to my songs and historical settings when I can. I looked in many places online to find info for the third grade version in your comments... that I am doing with my third graders. Your answers and comments were the most understandable as a whole, though not definitive - as is the 'dummy line' itself, I think. I like the version where it scared the animals and horses because of the noise of it cutting through farmlands. It works well with the train listening section that follows that has chickens and horses going wild - anonymous composer.
This is what I got online:

Definitions of Dummy Line on the Web:
a term used in railroad logging to describe railroad tracks that did not connect communities nor seem have any direction to them. Also called spur or tram lines.
www.longleafalliance.org/teachers/teacherkit/glossary.htm

another entry from online
The "Dummy Line"
In 1886, the Whitefish Bay Railway Company laid tracks which zigzagged through "Cementville" to connect to the city with the area that would be incorporated as Whitefish Bay six years later. The first trip of the railway's 12 ton locomotive, pulling three passenger coaches, did not bode well for the new company. The noisy engine terrified livestock and draft horses along its route, incurring protests from the owners of overturned wagons. In an effort to solve the problem, the company mounted a life size wooden horse onto a flatcar and coupled the car to the front of the engine to act as a calming vanguard. This experiment became known as the "Dummy Line" and continued to serve the area until 1898, when the Milwaukee Electric Railway and Light Company extended its service to Whitefish Bay and took over the route. http://www.villageofshorewood.org/train.htm