Steam Locomotive Classifications

[Anonymous]

Can anyone out there in railroad cyberland tell me the difference between the terms "Light" and "Heavy" when it comes to steam locomotives? For instance, what is the difference between a "light Pacific" and a "Heavy Pacific", "light Mike" and "Heavy Mile", etc. I have been attempting to get this question answered for over a year. Even my very detailed encyclopaedias of railroads do noit explain the terms. HELP!!!

[AltonFan]

Your question makes me think that you have in mind the standard locomotives designed by the USRA for use when the government controlled the railroads during and shortly after World War I. The USRA designed light and heavy versions of the 4-6-2, 2-8-2, 4-8-2, and 2-10-2 engines. The designs turned out to be so successful that after the railroads were returned to private control, a number of them bought engines based on these designs.

IIRC, the main difference between the light and heavy versions of these designs was the size of the boiler and the diameter of the cylinders. (I believe the driver sizes were identical between the light and heavy versions of the same type. I'll check my books when I get home.)

As far as I know, this is the only formal difference between light and heavy locomotive types, and it really only applies to the USRA engines, or the post-USRA copies of same. The Harriman standard locomotives might also have distinguished between light and heavy versions of certain locomotive designs.

AFAIK, there was no formal, universal definition of light and heavy locomotives such as one would expect to find in an edition of The Locomotive Cyclopedia. The reference would more likely to different classes of locomotives of the same type used by a particular railroad.

I hope this helps.

[AltonFan]

Okay, to follow up:

Generally speaking, the differences between the USRA light and heavy versions of the 4-6-2, 2-8-2, 4-8-2, and 2-10-2, were a larger boiler, firebox, and cylinders on the heavy versions. On the USRA 4-6-2 and 2-10-2, the heavy versions got larger drive wheels as well.

There were heavier versions of the 4-6-2, 2-8-2, 4-8-2, and 2-10-2 locomotives than the USRA "heavies" (Chicago, Minneapolis, and Omaha, a C&NW subsidiary, had the heaviest 4-6-2s; Great Northern, the heaviest 2-8-2, Illinois Central, the heaviest 4-8-2, and Santa Fe, the heaviest 2-10-2.)

And with the Harriman "Common Standard" types, the main distinctions seem to have been based on driver diameter.

[AltonFan]

One more follow-up:

According to George Drury's Guide to North American Steam Locomotives, the USRA designed the "light" versions of their locomotives to have a maximum load per axle of 54,000 lbs., while the "heavy" versions had a maximum load per axle of 60,000 lbs..