POLI 100K, Railroads and American Politics: Topic 1, Why are Railroads Important?

Railroads are more energy efficient and less polluting than trucks.
A diesel-electric locomotive can move one ton of freight an average of 379 miles (610 km) per gallon of diesel fuel.
This is 3 times the fuel efficiency of a truck.
Diesel-locomotives are three times cleaner than trucks on the basis of air emissions per ton moved.

Modern Diesel-Electric Locomotives are too large to be Manufactured on an Assembly Line. They typically weigh more than 200 tons (400,000 pounds) and range to 6,000 horsepower.

Diesel-Electric Locomotives come in two flavors -- Direct Current (DC) Locomotives, and Alternating Current (AC) Locomotives. These terms refer to the Traction Motors which turn the axles of the Locomotive. Until the 1990s DC Locomotives predominated because of their simplicity.

A DC Traction Motor is very old technology. They differ very little from the "modern" DC motor invented by Zenobe Gramme in 1873.

AC Traction Motor are simpler, more efficient, and more reliable than DC Motors. They do not have brushes or commutators so their physical structure is simpler. Consequently, they do not suffer from short circuits and are easier to maintain. The development of computer controlled solid state inverters capable ot converting DC to 3-phase variable-voltage variable-frequency power allowed the development of AC traction motors powerful and reliable enough to drive a Locomotive traction motor.

An AC traction motor consists of a stator (motor frame) and a rotating armature (rotor). Three-phase AC current is sent through the stator and electromagnetic induction rotates the armature. This consumes tremendous currents and powerful blowers are used to cool the motors.





It is the greatly increased Adhesion -- the percentage of a locomotive's weight on driving wheels that is converted into tractive effort -- of AC traction that gives it such a big advantage over DC traction. AC traction engines have achieved 45% adhesion and the maximum of a DC traction motor is about 30%. This means that pound for pound AC Locomotives can pull more weight than DC Locomotives. The reason adhesion is so high is due to the fact that AC motors fed from a single inverter (or synchronized multiple inverters) move in lock step. This improves adhesion because an individual axle encountering poor rail conditions will automatically lose torque as it starts to slip.

The General Electric's AC4400 Locomotive -- AC Traction at 4,400 HP




In contrast, the Union Pacific "Big Boy" 4-8-8-4 weighed 1,200,000 pounds (including the tender) and generated 6,200 horsepower.


The Chesapeake & Ohio's massive "Allegheny" 2-6-6-6 locomotives used to haul coal were reputably even larger than "Big Boy".