Throwback Thursday - Know Your GEMs

CN GR-17 number 4506 was resting in the Rectory St. shop area in London, Ontario early in 1988.

By Peter Mumby

Most railways group similar locomotives together by number series and class.  The system of classifying diesels evolved as various models were acquired and evaluated.  CN developed its classification system according to manufacturer, type of unit, and horsepower.

                                                                                                              The first letter in the class designation referred to the manufacturer.

G - the unit was manufactured by General Motors Diesel (or EMD in the US).

E - the unit was manufactured by General Electric.

M - the unit was manufactured by Montreal Locomotive Works or its successor, Bombardier.  Locomotives built by ALCO in the US were also designated by an M.

The second letter indicated the type of unit such as

R - roadswitcher; P - passenger; and S - switcher.

The digits indicated horsepower in hundreds.

Additional lower case letters indicated the order in which particular sub-groups were received.  Originally there was no indication of the number of axels, probably because a default of 4 axels was assumed.  In more recent times, the digit "6" has been added to the classification of modern six-axel power.  The CN classification system also applied to locomotives of GT, CV, and DWP. 

Here are the classifications of the three locomotives in today's photos:

CN 4506 is classed as GR-17h, meaning General Motors, roadswitcher, 1700 hp, and a member of the eighth group of these units acquired.

CN 3688 is classed as MR-18c, meaning MLW, roadswitcher, 1800 hp, and a member of the third group acquired.

CN 2514 is classed as EF-644a, meaning General Electric, freight, 6 axels, 4400 horsepower, and a member of the first group of these units.

It is interesting that, under the CN classification system, different looking locomotives could be in the same class.  For example, both the SW-1200 RS and the 4-axel GMD-1 were GR-12 units.

CN MR-18 number 3688 was teamed up with 3738 and 4530 at Windsor on June 14, 1986.

CN DASH 9-44CWL number 2514 leads 5780 through downtown London.