In my–admittedly limited–experience with railroads, it seems to me that the first measure that is used to describe the viability of a particular line is how many tons of traffic the line gets in a particular year. Why is this the standard (as opposed to car loads-per-year or gross revenue-per year)?
It seems to me that gross tonnage does not necessarily gauge other aspects of a line that are more important. For instance, NS’ Wabash main in Illinois/Indiana gets one heck of a lot of traffic, and by all accounts, I am told it is one of the gems of the NS empire in terms of profitability. Yet, because it hauls virtually no coal or iron ore, etc., its gross tonnage is not particularly high. If you look on the NS map that describes lines by tonnage per year, you would get the impression that this line was one of the lesser important lines in the NS system.
Before anyone accuses me of saying railroads do it wrong, that is not what I am saying. I am just saying I do not understand.
“Ton/miles” allows for a more positive spin on the relative economic health of the industry than would using “Revenue/miles”. Using ton/miles, railroads can claim 40% of US intercity traffic. But if they used revenue/miles, they could only claim about 15% of US intercity traffic.
I’ll take a guess at this and my answer should be considered opinion until corrected but most commodities shipped are billed by weight so the natural method of measuring would be by weight. Weight is easily converted into how many cars an engine or engines could haul so it is used to establish train length. Since a train exerts stress on the track infrastructure directly proportional to the weight passing over it ton miles are a good representation of the need to expend maintenance dollars on a given section of track. I would expect that those lines primarily carrying heavy ton miles of cargo like coal and iron ore as examples would require a much better base of ballast and heavier rail than one that would carry say passenger trains only.
Tonnage-and ton/miles- are good basic measures of production. The use of that data provides a fairly consistant record for comparing output, the cost of the output, and the resulting revenue of the output over different time periods.
Keep in mind that other data is reported. The AAR weekly reports carloads and intermodal loads with prior period comparisons, and that information is considered a quick measure of the level of business activity.
That’s pretty much it. Weight over a given streach of track for a given period will do x amount of “damage”. Tractive effort (measure of how much a locomotive can pull) is measured in terms of weight. Rates are figured in terms of weight because after you pull the car around (tare or empty weight) it costs more to pull a car with each pound added, and that is what the customer pays.
The ton mile figures you read are GROSS, not net. Total weight of the train and its contents. The customer pays the NET weight - what the cargo weighs.
Its also much, much easier to calculate. Just look at the Dispatchers Sheets. With modern computer systems it is now probably possible, but does the number really have any additional value? It would be worthless for judging track utilization, and maintenance requirements. On any given day a covered hopper car will hold a specific amount of grain by volume or weight, but the price of the grain will vary and probably the revenue too? Would you create a Railroad Rate Deflator Index so that you can eliminate the effects of inflation and make comparisons possible over time?