Although seemingly hidden away from the eyes of the public, there are thousands of private railways in the U.S. Most of these private lines are industrial in nature. Some of the major groups with private rail lines include agriculture, chemical, mining, power production, and the steel industry.
They take no passengers and the public has no reason to ever see them. Additionally, even if someone does see such a !ine, in many cases, there is no reason it should be recognized as anything private. Most Americans simply assume rail lines are public and there is very little written about the topic of private rail lines. Thus, private lines often go unnoticed.
In spite of the fact that private rails are typically quite short, usually just a few miles, many private lines have more than one locomotove in use. Although there are still lines that use cables, winches and pulleys to move their cars, gravity operations have largely fallen out of use. Most companies need more direct control over their equipment than this method provides. Thus, most modern locomotives are ones with an internal power source, often a diesel engine.
Believe it or not, just like a car battery can die and need a jump start, so can the battery of a locomotive. With a locomotive, no, it is not practicable to pull another engine up to it and use jumper cables to connect them, then give new life to the dead battery via a still live engine. So what does happen when some enormous locomotive engine dies? One answer is that a railroad locomotive starter can be brought in. This is a portable power supply to get the battery jump started again.
Time is money and private rail lines consist of very expensive equipment. Businesses simply cannot afford to let their private rails be idle for long periods. Thus, this is no time to make a phone call and wait for someone to show up. This is a time to have the power on hand that the company needs. It is not very different from using a lithium twin pack to do the same thing for a dead plane engine, a practice that seems to be more widely known.
The next time you see track in an odd place, remember that this might be a private line. They very often hide in plain sight.