The On-Line Magazine of Rideable Model Railroading

  NUMBER 184


© March 25, 2012   

 ©Discover Live Steam. This material may not be published, rewritten, or redistributed without written permission.

 | Read other articles Send in an article |

Classification of Diesel Locomotives

Photo by Fredrick Smith ©Discover Live Steam.

Written by Dan Foltz


The classification of locomotives has been based on the number of axles or wheels for many years. The Whyte wheel system was used on steam engines and counted wheels.

"The Whyte notation for classifying steam locomotives by wheel arrangement was devised by Frederick Methvan Whyte and came into use in the early twentieth century encouraged by an editorial in American Engineer and Railroad Journal (December 1900). Whyte's system counts the number of leading wheels, then the number of driving wheels, and finally the number of trailing wheels with groups of numbers being separated by dashes." 1

An example of a Whyte classification of a steam locomotive would be the 4-4-0 locomotive which has 4 leading wheels, 4 driving wheels and zero trailing wheels.

Diesels follow in much the same way, but using axles. 

"The AAR system counts axles instead of wheels. Letters refer to powered axles, and numbers to unpowered (or idler) axles. "A" refers to one powered axle, "B" to two powered axles in a row, "C" to three powered axles in a row, and "D" to four powered axles in a row. "1" refers to one idler axle, and "2" to two idler axles in a row. A dash ("Ė") separates trucks, or wheel assemblies. A plus sign ("+") refers to articulation." 2

The number of powered axles is designated by letters:

1 axle = A

2 axles = B

3 axles = C

4 axles = D

5 axles = E

6 axles =F

7 axles =G

Switch engines with two axles are designated A A and with three axles A A A. If a truck is on the diesel, then it would be a B. A switch engine with two trucks would be a B B.

Take a look at the Pennsylvania GG 1 (below) and count axles.  It's classified as a 2-C+C-2.

Photo by Bill Shields ©Discover Live Steam.


GP for diesels is a general purpose diesel and a SD is a special duty diesel (photo at the top of the page). Usually the GP has a two axle truck making it a B-B and the SD has a three axle truck. making it a C-C. Keep in mind that in any system there will be variations.

Also, if you look at a diesel and see a letter F on it, that tells the engineer which end is the front end. You donít need that on a steam engine. You know which end is the front end.

Switch engines started to be built in the early 20ís and 30ís. They were first used in yards for switching and in cities to eliminate soot and dirt caused by the steam engines.

The diesel engine was developed by Rudolf Diesel in 1893.  The builders of the diesels were ALCO, GE, GM (EMD), BALDWIN, FAIRBANKS-MORRIS.  The word "Diesel" has stuck as a name for the machine, which is not exactly what it is. The correct term for this locomotive is a diesel-electric. The diesel engine operates a generator that makes the electric for the motors that operate the axles. 

Today, when talking about diesels, the designation of a unit usually is up to the builder as to what the letters and numbers all mean.

Diesels have replaced steam engines mainly due to maintenance costs. A steam engine usually got serviced on a regular basis such as every 400 or 500 miles of use. The diesel could run for 60 or 70 days with just a check-up service. In addition to less frequent maintenance, fuel for the diesel usually was a lot cheaper compared to the fuel for a steam engine.

The cost of operation put the steam engine out of use and brought the diesel in.

Today, we see other countries operate passenger trains at speeds up around 200mph.  The United States might get a train to go that fast someday. The US Congress has declared a 78mph speed limit on trains in this country, however. The only monorails in the US are operated by Disney, whereas some other countries already operate monorail passenger service. It is hard to say when train service in the US will be equal to other countries in the world.

1 Whyte Notation

2 AAR Wheel Arrangement


Written by Dan Foltz

 | Read other articles Send in an article |


the end

Would you like to discuss
the ideas in this article? 
Post a comment or question here.

Write to
(the subject line must contain the word discover)


Have an idea for an article? 
We need your article on ....

Technical Issues such as problems and solutions associated with steam locos, hydraulic drives, electric drives, track laying and maintenance, signal systems.

Construction Projects, mostly looking for car projects (let's leave the steamer building to the print magazines). How about scenery construction or building a hand car?

Full scale railroads and museums.  If you work for or volunteer for a railroad, if you've visited one recently and have a few photos and can write up a half dozen paragraphs on it, we'll be happy to put it up on the web.

Live Steam Railroads. How about a little background and a tour of your railroad or one you've visited.

Please share what you know with us.


The On-Line Magazine of Ridable Model Railroading

Read Other Articles

| Live Steam Railroads | Suppliers | Postings | For Sale  |
 | Events Calendar | Books | Magazines | Videos  | Photo Contest |


Hit Counter