Introduction to Railroads

by David S. Lawyer More railroad articles by David Lawyer


1. Copyright

2. Introduction

3. History

4. Motive power & Locomotives

5. Cars

6. Tracks, Yards & Signals

7. Yards

8. Operating Characteristics

9. Further Reading:

10. Key Terms

1. Copyright

Copyright 1995-2005 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. Most of this was written in 1995 and hasn't been updated much (except to improve clarity and for fixing typos, etc.).

2. Introduction

The most energy-efficient motorized land transportation system known to man is the operation of trains on a railroad. While trains are less significant today than in the past, in the US they still haul more ton-miles of freight than trucks do and hold great potential for the future. 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 which usually provides faster (but more costly) service. Exceptions include new automobiles (which are shipped by train in special cars), and containers (including truck trailers) which ride "piggyback" on railroad flat cars.

3. History

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.

3.1 US history

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 only about 1% of the passenger traffic in the US, 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. Today railroads transport about 40% of the intercity ton-mi as compared to about 30% for trucks. But trucks receive several 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.

3.2 Foreign Developments

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. 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 railroads. Russia (and later the communist Soviet Union) and Red (Communist) China chose railroads. Thus Russia and Asia 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".

4. Motive power & Locomotives

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.

4.1 Diesel-electric locomotive (or diesel locomotive)

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.

4.2 Electric Locomotive

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.

4.3 Locomotive Braking

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.

5. Cars

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.

5.1 Freight Cars

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 are used for moving containers, truck trailers and large machinery. The tank car is a cylindrical tank for moving liquids such as chemicals and petroleum products.

5.2 Passenger cars

Passenger cars offer more space for a passenger than the automobile. Coach cars contain seats much like a bus. Sleeping cars 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. Many 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.

5.3 Car brakes

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.

5.4 Couplers

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.

6. Tracks, Yards & Signals

6.1 Track

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.

7. Yards

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.

7.1 Signals

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.

8. Operating Characteristics

8.1 Speeds

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 has been estimated to be about 5 mi/hr in the 1930's. It was still at about this speed in the 1970's but likely increased in the 1980's. The main cause of this slow speed is delay at yards. Piggyback 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.

Average freight train speed remained at about 20 mi/hr since the 1960's but during the 1980's increased to almost 24 mi/hr. This low speed is 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.

In order for railroads to regain large amounts of freight lost to trucks, they need to reduce yard delay and reduce the number of shipments arriving late. 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 such shipments.

Passenger trains speeds have averaged over double that of freight trains. This meant 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).

8.2 Energy efficiency

Today the oil reserves underground in the US are becoming depleted resulting in the US importing over half of the oil it consumes. 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 is a few times more energy efficient (more ton-miles of freight hauled per gallon of fuel) than the highway truck.

Passenger trains today are (on average) only somewhat 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 much more energy efficient than the auto. But diverting truck freight to railroads by improving service may have even greater potential for energy savings.

9. Further Reading:

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.

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.

10. Key Terms

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

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 trains run

Rolling resistance--The force opposing the forward motion of a train (or other wheeled vehicle) mainly due to the rolling of the wheels

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