mailto:email@example.comMore railroad articles by David Lawyer
Copyright 1995-2017 by David S. Lawyer. Feel free to make copies but commercial use of it is prohibited. For example, you can't (except to an insignificant degree) combine it with advertising on the Internet. Please let me know of any errors or suggestions for improvement. Much of this was written in 1995.
Railroad were the first significant form of motorized land transportation starting in 1825. Since the steel wheel on steel rail has much lower rolling resistance per ton of vehicle weight than other transportation modes, one would expect railroad trains would be more energy-efficient than other types of transportation. But other factors, such as trains being overly heavy and airplanes improving their efficiency and filling their seats, results in both airplanes and long distance busses being more energy efficient than passenger trains in the United states. But for freight transportation rail is usually more energy thrifty.
While trains are less significant today than in the past, in the US (as of 2017) they still haul more ton-miles of freight than trucks do (see AAR data), although much of what they haul consists of bulk commodities such as coal. A train consists of a series of railroad cars with steel wheels running along the steel rails of a railroad track. The power to move a train usually comes from one or more locomotives at the head of the train pulling the cars behind it. Locomotives in the US are usually powered by diesel engines but elsewhere (especially in Europe) many locomotives are electric and obtain electric power from an overhead wire strung above the track. In the US, railroads are primarily used for hauling freight, especially lower valued bulk commodities such as coal, lumber, food, paper, chemicals, and metals. Higher valued manufactured consumer goods and small shipments of less than a carload are more likely to be moved by truck or by by large containers which fit both on trucks and railroad cars. Truck transportation provides service to location where there is no railroad access, and where there is railroad access trucks are usually faster but generally more expensive. Due in part to the wages of the truck driver. Computer driven, driverless trucks in the future may bring down truck costs, resulting in even more diversion of rail freight to trucks.
The railroad was invented because it could save energy at a time when energy costs were high. Before the invention of the steam railroad, animal power was used to pull carts along dirt roads and this animal power was quite expensive. Long ago it was found that much less energy (and fewer animals) were required to pull a cart if the wheels of the cart were run along wood boards instead of on a dirt (or rough rock) surface. The wood boards became wood rails and the wheels were designed so that they would stay on the rails. The first major railroad (using steam power instead of animal power) was built in England in 1825. In 1830, the first railroad was built in the US. Railroads were often a great success and the railroad building boom began in Western Europe and in the eastern part of the United States.
By the time of the Civil War around 1860 , an extensive railroad network had been built in the US, mostly east of the Mississippi River. The success of the railroad had put many a canal or toll road out of business. The railroads still mostly used wood rails but iron straps were attached to the tops of the wood rails for a metal running surface. Locomotives often burned wood instead of coal to generate steam. Railroads remained king of land transportation until the development of the automobile and truck in the early years of the 20th century.
In the 1920's, a network of paved highways was constructed throughout the US and automobiles and trucks competed with trains. An auto trip provided much more freedom than a train trip since one could stop whenever and wherever one chose as well as make side trips to points of interest. Auto travelers never had to concern themselves with the delays and inconveniences inherent in schedules: waiting for trains, missing trains, and changing trains.
Today trains transport under 1% of the passenger traffic in the U.S. having lost most of their former passenger traffic to the auto and airplane. In the 20 century, the government greatly improved navigation on various rivers such as the Mississippi. This, as well as the building of pipelines to transport oil, gasoline, etc. resulted in additional loss of railroad freight traffic. In the year 2000 railroads transported about 40% of the intercity ton-mi as compared to about 30% for trucks per TiA. But in the early 21st century (after 2000) the modal split between rail and truck reversed (if the BTS estimates are to be believed, and today (2017) trucks haul significantly more ton-miles than rail. Trucks receive many times as much revenue as railroads because they provide faster service, move traffic door-to-door, and accept small shipments. In most cases, shippers who ship by truck have no other reasonable choice.
The steam locomotive (which was only about 5% efficient in converting heat energy into power) was replaced by diesel locomotives in the late 1940's and early 1950's. Diesel is about 4 or 5 times more efficient in converting the heat content of fuel into energy used to move a train. In spite of improved fuel efficiency, low gasoline prices and improved highways contributed to the continuing decline of rail. Another contributing factor was labor union rules such as requiring the operation of trains by 5-man crews which were paid a full day's pay for 100 miles of work. Both management and government failed to correct these and other railroad inefficiencies including an overly complex rate structure.
In the 19th century, most of the railroad development occurred in the industrialized countries of North America and Europe. Africa, and South America only weakly developed railroads. Asia, although backwards in industry, was in some ways an exception. The British built many railroads in their Asian colony of India, and Russia built railroads into the Asiatic part of Russia and in China.. In the 19th century, the option of establishing a highway system for autos and trucks didn't exist since there were no autos or trucks.
In the 20th century, countries had a choice between developing highways and/or railroads. Russia (and later the communist Soviet Union) and Red (Communist) China chose to emphasize railroads. Thus Russia and Asia (including China) wound up with a heavily used railroad system. Prior to its demise, the Soviet Union hauled half of the world's freight ton-mi. In Africa and South America, the choice was often in favor of highway development resulting in weakly developed railroad transportation.
Most railroads in the US were organized as private companies and remain so today. However, most passenger rail transportation in the US today is run by government (or quasi-government) agencies. In other countries, railroads were often started as private enterprises that were later taken over by the government. The government-owned railroads in non-communist countries were usually operated at a financial loss. In recent years, in order to avoid such losses Japan and Britain have sold their railroads to private companies in a trend called "privatization".
It takes energy to move a train and this energy is supplied by motors. The motors that directly drive the wheels of any locomotive are almost always electric traction motors. In some passenger trains, the electric motors are underneath the cars in which passengers ride. For an electric train, the electricity comes from an overhead wire above the track or from a third rail which conducts electricity. Third rail is often used for urban rail mass transit. In a diesel locomotive, the electricity is generated onboard the locomotive by a diesel engine driving an electric generator. A train often has more than one locomotive to supply power but the front locomotive usually controls the operation of the other locomotives. There are two basic types of locomotives today: diesel-electric and electric.
Almost all U.S. locomotives are diesels burning liquid diesel fuel. The diesel motor (engine) drives an electric generator which today is usually an "alternator" since it generates alternating current (AC). Then diode rectifiers convert the AC to direct current (DC). If AC motors are used to turn the wheels, the DC will be converted back to AC at frequencies which vary with the speed of the motor (and speed of the train). This is done by a power electronic device called an "inverter" which does the inverse of what a rectifier does.
One may also think of this as a diesel motor with an "electric" transmission to get the power to the wheels of the train. The electric transmission permits the equivalent of a large number of gear ratios and provides a simple method of transmitting power (via electric cables) to each the many driving wheels of the locomotive. Each axle (which has a pair of wheels attached to it) usually has its own electric motor to drive it.
There are two basic types of railway electrification (and types of electric locomotives): AC (Alternating Current) and DC (Direct Current). Most urban rail transit uses DC to run trains without locomotives, each car having its own motors.
In order to have an efficient power distribution system, a much higher voltage is desirable in the overhead wire than is required to operate the motors. Thus modern locomotives contains devices to reduce the voltage to that suitable for the motors. Furthermore, as the motors turn slower, they need less voltage. AC systems use a transformer to reduce voltage and then diodes to rectify it to DC. Modern DC systems use solid state power electronics to reduce voltage. Older DC systems often wasted power in resistors to reduce voltage. DC voltages are usually 600 to 3000 volts while AC is usually 12,000 to 25,000 volts. With 3000 volts DC it is possible to put 4 traction motors in series and get only 750 volts on each motor.
Note that DC is lower voltage than AC. This was because at the time most railroads were electrified, there was no power electronics to reduce the voltage. In the future, it may be feasible to employ higher DC voltages for main-line electrification. A factor favoring DC is that electric currents create magnetic fields to which people are exposed. It is suspected that exposure to AC fields may increase the risk of leukemia in children. DC currents create a steady magnetic field (as does the magnetic field of the earth) and are thought to pose no hazard to health (at the levels found near an electric railroad).
While the electric locomotive is quite efficient in converting electricity into traction force at the wheels, one must multiply this efficiency by the thermal efficiency of generating the electricity at a stationary power plant. Also there are losses in the wires which carry electricity to the train. When all factors are taken into account, the overall efficiency of an electric locomotive is only a little better than a diesel unless there are many downgrades where the electric locomotive can generate "free" electricity. A major advantage of electric trains is that power plants may use a wide variety of fuels such as coal to ultimately power trains.
Not only does the locomotive provide power to move the train, it also absorbs energy to stop the train. An air compressor in the locomotive provides compressed air to the cars in the train and this air is used to apply their brakes. Thus they use "air brakes". Also, the traction motors may be connected electrically so that they become generators. This creates a braking force at the driving wheels of the locomotive but something must be done with the electricity generated. One way to dump this electric energy is to put current through large resistors in the locomotive and thus convert the energy into waste heat. This is known as dynamic braking. It saves much wear on the brake shoes of a train.
For an electric locomotive the electricity may be put back on the overhead wire provided there are other trains nearby that can use the energy. In some cases it can even be transferred back to the power system to supply homes, etc. with electricity. This is called regenerative braking. In order for power to flow from the locomotive to the overhead wire, the voltage output from the locomotive must be kept slightly higher than the normal voltage on the overhead wire. Not all electrically powered trains have regenerative braking.
Modern railroad cars, like most automobiles, are usually suspended by coil springs. But unlike the automobile, the wheels are all-steel. A pair of wheels are mounted on each end of a rotating axle. Two such wheel pairs are contained in a "truck" which also contains the coil springs. The truck is free to rotate as the car goes around a curve. Model trains also use trucks.
There are many types of freight cars. A box car is completely enclosed with large doors at the sides. It can carry all types of general merchandise and other goods packed in boxes, bags or barrels. Gondola and hopper cars are for granular bulk commodities such as coal, grain, and granular chemicals. Hopper cars have doors underneath the car which may be opened so that the granular freight just falls out to unload it. Flat cars are cars with a flat bed but no sides. They were once often used for moving containers, truck trailers and large machinery, etc. But the well car (actually 3 or 5 cars permanently coupled together) is used today for hauling containers. The floor is recessed down low (like a well) so that containers can be double-stacked, one on top of another. The tank car is a cylindrical or oval tank for moving liquids such as chemicals and petroleum products.
Passenger cars offer more space for a passenger than the automobile. Coach cars contain seats much like a bus. Sleeping cars may have seats that convert into beds for overnight travel with upper berths on the walls. The result looks something like a bunk bed. Some cars are divided into rooms called "compartments". Dining cars are like restaurants. Most passenger cars have restrooms. Baggage cars not only carry the baggage of passengers but also may carry express packages. Many years ago special cars to carry mail ran in US passenger trains. Some cars are bilevel like a two story house and can seat over 150 passengers.
The brake shoes may be seen the on sides of the wheels. They are pushed against the wheel by one or more brake cylinders which are powered by compressed air. The compressed air comes from a reserve supply kept in a tank (or two) under the car. These tanks are kept full by air supplied from an air compressor in the locomotive via a brake pipe. Each car has a flexible rubber brake hose at each end of it which must be manually connected to the brake hose of the next car. This hose supplies compressed air to the car.
The brake system is designed "fail safe" so that if the train should break in two, the air hoses will pull apart and release the pressure in the main brake pipe which will cause the brakes to apply. To apply normal braking, the locomotive driver reduces the pressure in the main brake pipe which runs the length of the train.
At the end of each car is an "automatic" coupler invented in the 19th century. It doesn't couple brake hoses or any electrical contacts. It uncouples by manually pulling a lever. In Western Europe the couplers are not automatic and the cars are manually coupled together by large screws [after around 2000 many European passenger trains use the Schaku fully automatic coupler]. Russia has more advanced automatic couplers than the US since they are always ready to couple. In the US, at least one coupler must have a knuckle open for coupling to take place.
The track consists of 2 steel rails attached to wooden (or concrete) ties which are laid in a bed of crushed stone known as "ballast". The distance between the rails is known as the gauge. While a highway must support wheels that may run anywhere on the pavement, a railroad only supports the wheels along two lines where the rails are located. These locations must thus be built to support very heavy loads. The rail accepts the high load from the train wheels and, like a steel I-beam, distributes the load to the ties below via steel tie plates. The ties further spread the load to the ballast layer which further distributes the load to the ground. Thus the pressure on the ground is far less than that under a train wheel. If the rail did not bend a little as a train wheel runs over it, it could not spread out the load to the ties below. Thus the support for the rails (such as wooden ties) should be flexible to allow for such bending.
A railroad yard is like a large train station for freight. Yards consist of many railroad tracks in parallel where cars change trains (see fig. 5). At a yard, a train arrives to be broken down into individual cars (or groups of cars coupled together and going to the same destination). Taking apart a train is time consuming since each car must have its brake hoses manually disconnected, air bled from the brake cylinders, etc. In a "hump yard", arriving cars wait to be pushed to the top of a hill known as the hump. A car coasts down a track which has many switches in it so that, depending on how the switches are set, the car winds up on a certain track. When enough cars have accumulated on one or more such tracks to make up a certain train, then the cars from those tracks are pulled out and put together into a new train. This entire process may delay a car several hours, and in some cases, over a day or two. Yards are becoming less significant today as much traffic consists of container trains (that seldom use yards) and unit trains that transport say a whole trainload of coal from mine to an electric power generation plant.
Railroad signals seem somewhat like signals for autos at intersections but serve different functions on a railroad. Trains cannot stop as quick as autos can so signals are used to maintain a safe distance between trains. On a single track section of line, the signals must prevent head on collisions between trains going in opposite directions. The color of a signal sets the speed limit with a red light meaning "stop here" (or don't enter this track). Switches are often remotely operated from hundreds of miles away so that a train may take to a side track to allow another train to pass it. The locomotive often has a miniature signal inside it so that the train driver can always see it. Electric current in the rails is used to send codes to the locomotive to operate this "cab" signal. The location of a train is often detected electrically since if a train is present the metal wheels and axle electrically connect (short out) the two rails.
Unlike the former Soviet Union which established a uniform system of signaling, in the US each railroad company has it own signalling system and rules. Thus a locomotive which runs on different railroads needs to have equipment to detect the various types of electrical signals in the rails.
One may roughly partition freight train transportation into three basic types:
There are some situations where a large volume of freight flows between the same origin and destination such as between a coal mine and an electrical generation plant. A "unit train" just uses one train to haul such fright between the origin and destination sidings. For example, it may haul a trainload of coal from a coal mine to a power plant to generate electricity. Railroads also run what they call shuttle trains which are similar to unit trains but usually run for shorter distances and will allow more flexibility like varying the number of cars in the train and charging the shipper accordingly.
Unit trains (including shuttle trains) travel in one direction fully loaded and move empty in the opposite direction. This is obviosly not as efficient as the case of hauling goods in both directions as can happen with some carload traffic as described below. Unit trains generally haul only bulk commodities such as coal, petroleum (oil), sand, grain (food), and mineral ore.
A freight car is loaded with freight at a siding track and then picked up by a local train which takes the car to a yard (or the like) where it is eventually joined to a longer-distance train. If this train doesn't go to the destination region of the car, then the car is let off at a yard where after a wait of perhaps a day, the car is put on a train that gets the car significantly nearer to its destination, etc., etc. Finally the car is put on a local train that takes it to its final destination siding for unloading. In some cases there are a few adjacent cars on the originating siding that are all going to the same destination siding. So then these same-destination cars can all remain coupled together during their trip. Such a group of cars is known as a "cut". Prior to the 1960's, almost all freight traffic was of the "carload" type.
Bear in mind that the above definition of "carload" is not standard and that unit trains also carry "carloads" of freight.
This involves a container the size of a truck trailer. A truck hauling such a container looks like a typical semi-truck (big-rig) but the semi-trailer of this truck is really just a large container set down on a truck chassis. The container provides the truck trailer but the truck provides the wheels for the container to roll on (as well as a steel framework under the container).
The container trains usually don't go get sorted at yards and thus service is much faster than carload. The originating and final sidings may be called an intermodal facility where the containers from trucks are loaded/unloaded onto/from trains. that go by sea on ships, there are some facilities for directly loading containers from ship to train (and vice-versa) but it's mostly done by using trucks to move containers to-from a rail yard near a port to-from a ship-side dock. This is obviously not the most efficient way to do it.
In the European Union, the definition of "intermodal" excludes the transfer of bulk goods such as coal from one mode to another. Some might claim that transferring coal from a train to an ocean-going ship is "intermodal" since it involve two modes: rail and ocean ship.. But this transfer is normally made in two steps: 1. coal from the train is offloaded to storage at the port. 2. coal from storage is transported by a portable conveyor belt to load the ship with coal. So in a sense the ship is taking on coal that happens to be stored at the port and there is no direct transfer of coal from the train to the ship. Is this intermodal? Maybe yes, but per the common usage of "intermodal" probably no.
Rail-truck competition resulted in diversion of freight from carload to truckload so carload freight declined. But intermodal (container) service grew because it's cheaper than trucking and not too much slower. Unit trains were helped by increased coal traffic for electricity generation but there are environmental objections to the use of coal. However the sum of Intermodal and unit train transport increased more than the decline in carload transport. The result is that that while total rail traffic has grown over the past few decades, truck traffic (in ton-miles) has grown even faster and has now (2017) become significantly greater than rail.
Since railroad resistance is low, the optimal speeds for trains to travel at is often significantly higher than autos and trucks on a highway. In actuality, rail speeds are often lower than highway speeds. As speed increases, so do the energy costs.
The low speed (and variability of speed) of railroad freight shipments has resulted in trucks hauling freight where speed and reliability of arrival time are important. Although it may seem ridiculously low, the average speed of a loaded railroad freight car was estimated to be about 5 mi/hr in the 1930's and was reported to still be about this slow in the 1970's. The main cause of this slow speed is delays at yards. Intermodal trains and unit trains (which haul an entire trainload of something from point A to point B) avoid yard delays and thus make much better time. The average speed of freight trains in the 21st century is a little over 20 mi/hr when the train is moving (neglects delay in yards).
Average freight train speed remained at about 20 mi/hr in the 1960's but during the 1980's increased to almost 24 mi/hr. This low speed was due to the many stops trains make, especially at signals on single track (predominates in much of the US). When two trains on such a track are headed toward each other, one must take to a siding (where there are 2 tracks for a short distance) and wait for the other train to go past.
Reducing yard delay and reducing the number of shipments arriving late would improve the quality of railroad service.. This involves the complex problems of changing labor work rules so as to be able to run more trains, and changing the technology of sorting cars at yards. Developing the ability to efficiently handle small shipments would earn a very high revenue per ton-mile for such shipments.
Passenger trains speeds have averaged over double that of freight trains. This meant passenger train speeds of about 35 mi/hr in the 1930's, increasing to 40 mi/hr by the 1950's. In the 1970's, after Amtrak was created by the government to handle intercity passenger trains, Amtrak's average speed rose to over 50 mi/hr. This was because slower trains were discontinued and the slower commuter trains (averaging only a little over 30 mi/hr) did not become a part of Amtrak. Amtrak trains often reach over 100 mi/hr between New York City and Washington, DC. High speeds make passenger rail transportation more competitive but increase energy consumption due to increased aerodynamic drag (see below).
Today the oil reserves underground in the US are becoming depleted resulting in the US importing in 2016 about 1/4 of the oil it consumes. But back in 2005 it was 60%. Also, the more oil burned in transportation, the greater in increase in global warming Thus energy efficiency is important. Trains have much less "rolling resistance" (the force opposing the forward motion of the train when rolling at low speed) and less "aerodynamic drag" (wind force) than autos or trucks. While the locomotive front end of many trains is not well streamlined, each car in the train helps shield the car behind it from wind force, thus reducing aerodynamic drag. Aerodynamic drag increases as the square of the velocity and thus becomes very significant at high speeds but is less important for freight trains operating at low speeds. Thus the freight train in most cases is significantly more energy efficient (more ton-miles of freight hauled per gallon of fuel) than the highway truck. But efficiency is dependent on what kind of goods are being hauled. A heavy freight car hauling lightweight low density consumer goods may use more energy than hauling them by truck.
Passenger trains today are (on average) only a little more energy efficient than the automobile although during World War II diesel passenger trains got about 100 passenger-miles per gallon. This unexpected poor showing for the passenger train today is in part due to the great improvement of automobile miles-per-gallon during the 1980's. Smaller auto engines (and smaller autos) provided less acceleration but better efficiency while still permitting speeds at the speed limit. Passenger trains had no such options for improved efficiency since they were never overpowered or oversized as much as autos were. Long distance passenger trains tend to weigh much more per seat than the auto, since they provide each passenger more space. Commuter trains taking people to work have more seats and thus weigh less per seat but waste energy by making frequent stops. Autos use waste heat from the engine for heating while passenger trains often use inefficient electric heat. The percentage of seats occupied on trains is less than many people think, often under 50%.
The higher energy efficiency of passenger trains outside the US shows that there is potential for improvement. Streamlined passenger trains operating on the surface (and not underground where aerodynamic drag is higher) at moderate speeds with fewer stops can be more energy efficient than the auto. But diverting truck freight to railroads by improving service may have even greater potential for energy savings. But the prospect of driverless trucks works against this.
Rail Carload Traffic (presentation)
Armstrong, John H. The Railroad, What It Is, What It Does. Omaha: Simmons-Boardman, 1990.
Association of American Railroads, The Car & Locomotive Cyclopedia. Omaha: Simmons-Boardman, 1984, 1996.
Association of American Railroads (AAR), Class I Railroad Statistics (on Internet) For past years see "Railroad Facts" or "Yearbook of Railroad Facts"
General Railway Signal, Elements of Railway Signaling. Rochester, New York: General Signal Corporation, 1979.
Hay, William W. Railroad Engineering. New York: Wiley, 1982.
Federal Railroad Administration, The Railroad Situation. Washington, DC: US Government Printing Office, 1979.
Stover, John F. American Railroads. Chicago: University of Chicago Press, 1961.
Wyckoff, D. Daryl, Railroad Management. Lexington, Massachusetts: D.C. Heath & Co., 1976.
Ballast--Gravel (or crushed stone) in which the ties of a railroad track are put
Car--A steel wheeled railroad car, many of which are coupled together to form a train. There are many different types of passenger cars and freight cars
Diesel (or Diesel-Electric) Locomotive--A locomotive where a diesel engine-generator generates electricity to power electric motors which turn the wheels
Electric Locomotive--A locomotive which obtains electricity from an overhead wire and uses electric motors to drive the wheels
Intermodal--Moving containers that only use rail for part of their journey. The other part is by boat or truck
Passenger-mile--A unit of measurement of passenger transportation representing the hauling of one passenger one mile
Rails--Metal beams, supported by ties, on which train wheels run
Rolling resistance--The force opposing the forward motion of a train (or other wheeled vehicle) mainly due to the rolling of the wheels
Siding--An extra railroad track(s) that usually runs parallel to the main track(s) used for trains passing each other or used for loading, unloading, or storage of railroad cars.
Switch--A fork in railroad track where a train has a choice of going straight ahead or veering off on a side track
Ties--Wooden (or concrete) beams laid in the ballasted ground to which the rails are attached with spikes
Truck--The set of 2 axles (each with 2 wheels) including coil springs and steel framework which supports half of a railroad car
Ton-mile--A unit of measurement of freight transportation representing the hauling of one ton of freight one mile
Yard--Several (or more) parallel railroad tracks (with many switches) used for sorting railroad cars so they can be made up into trains
Unfortunately, the annual publication "Transportation in America" (TiA), A Statistical Analysis of Transportation in the United States, ceased publication in 2002 (which covered data only up to 2000). It's tables of domestic intercity ton-miles and passenger-mile by mode have not been replaced in any other publication. The key word here is "intercity". For example, the US Bureau of Transportation Statistics does publish (every 5 years) estimates of truck ton-miles but unfortunately they are for all truck movements, both local and intercity trucking. To compare truck transportation to rail frieght, one should look at the intercity market only since frieght trains don't haul good locally in an urban area and thus one needs an estimate for intercity truck tranportation. Also, it's nice to have yearly estimates and not just ones every 5 years.
There is a major disceptancy in figures for ton-miles by rail for 2012 (the latest year reported as of 2017). The Association of American Railroads (AAR) reports 1.71 trillion ton-miles for Class I railroads (which omits all traffic on Class II and III railroads. (Class I are those road that have annual income of over about a half-billion dollars.) However the Statistical Abstract of the United States reports 1.21 trillion for non-intermodal rail and shows .17 t. for truck-rail intermodal, plus .03 for truck-domestic-water intermodal (moslty on inland canals not deep-sea). Assuming that rail consumes about 90% of ton-mi of truck-rail intemodal ton-mi and 1/3 of ton-mi for rail-water intermodal, then the total ton-mi for rail is 1.37 trillion ton-mi. But this is less than the 1.71 reported by the AAR for class I only, Instead of the 1.37 being less than the 1.71 it should have been greater since it presumably indcludes Class II and III ton-mi.
The Bureau of Transportation Statistics (BTS) also shows data for 2007 showing 1.52 trillion ton-mi for rail, which is still lower than the AAR's 1.71 but higher than the Statistical Abstracts's 1.37. So the BTS figures may be in error since they are based on samples while the AAR figures are not based on sampling but are the actual data reported by the railroad companies. More investigation into this discrepancy is needed.