What is the purpose of a railroad or any other freight carrier?
To collect raw material to be converted into work in process, then into finished goods to be distributed to the consumer. Of course, as the material goes through each stage it increases in value and usually decreases weight.
Examples: Iron ore into iron ingots into coil steel into fenders into cars or trucks to the consumer.
Wheat into flour into bread to the consumer.
Some rail lines were built for gathering. Lines for gathering from farms are referred to as Granger railroads. Trivia: What state had the most amount of track in the year 1900? Iowa, for collecting grain.
Other collecting rail lines were built to collect iron ore or wood chips or crude oil.
Some rail lines were built for dispersing finished goods. Panama City, Florida was about 100 miles from any main line railroad, so a line was built with the purpose of bringing in finished goods.
There is another type of line that is used for connecting major traffic centers. In the south, the Mississippi River is a major problem for east-west traffic because there are such few rail bridges. There is one major bridge at New Orleans causing a major bottleneck. However, the Kansas City Southern is a major user of bridge at Vicksburg, Mississippi, with a line connecting Meridian, Mississippi to Dallas, Texas. Nearly all of this line's traffic is going from end to end, coming from another railroad, going on to a different railroad. This is called overhead or bridge traffic. (Now Norfolk Southern has trackage rights over this line.)
Any examples given for the type of rail line takes into account that there are other uses for the rail line. Panama City is now a seaport, so that line also acts as a bridge rail line, because the goods are delivered to the ships.
Each railroad is a small monopoly because it directly connects a supply source (wood chips) to a user of the material (paper mill). But because there are so many paper mills, the railroad