Tonnage reduction with temperature falling

NW Mailing List nw-mailing-list at
Mon Dec 17 16:59:32 EST 2018



Good info. Knuckles more prone to break in cold weather . . . so reduce tonnage. Sounds like a reasonable factor to me. Is this the experience of those on the list who operated?


I don’t see how cold temperatures would create “Lower steam pressures means less usable energy in the available steam”. I don’t believe NW steam locos would have run at lower than standard operating steam pressures due to cold weather. It may have taken a little more coal to achieve and maintain operating pressures but that would not have required tonnage reduction, maybe just an extra stop for coal.


John Garner, Newport VA


From: NW Mailing List [mailto:nw-mailing-list at] 
Sent: Monday, December 17, 2018 10:39 AM
To: NW Mailing List <nw-mailing-list at>
Subject: Re: Tonnage reduction with temperature falling


In the steam era, thermodynamic properties would have come into play.  Available heat generation is limited by the size of the firebox which is held constant, starting with lower water temperatures takes more heat input to get up to designed steam pressures.  Lower steam pressures means less usable energy in the available steam.

But these limitations were most likely multi-pronged.  In diesel years the colder temperatures lower the available pressure of the air in the brake system (also an issue during the steam era).  The compressors on the lead locomotives will wear themselves out trying to maintain brake pipe pressure to the point you start bursting rubber hoses which are already made more brittle by the lower temperatures..  Putting locomotives mid- or rear-train will help alleviate this.  Canadian National (CP possibly too) has mid-train heater cars that they run in the winter to help keep the brake lines warm and charged.

Reducing tonnage is a common solution for railroads in winter.  I read an article several years ago where the DMIR was having issues with breaking couplers during winter months due to the ambient air temperature and wind chill combining to take the steel below it's ductile-brittle transition temperature.  The DMIR solution was to draw-bar cars together where possible and to reduce tonnage.  It also helps that they don't run as much in the winter months due to Lake Superior being frozen over and ships not being able to access the ore docks.


Josh Blevins 

Charlotte, NC


On Monday, December 17, 2018 8:35 AM, NW Mailing List <nw-mailing-list at <mailto:nw-mailing-list at> > wrote:



On 12/16/2018 10:01 PM, NW Mailing List via NW-Mailing-List wrote:

I was under the impression that the reason the tonnage went down with temperature was thermodynamics of water evaporation to get the needed pressures of steam. The point about oil and grease viscosity makes good sense as well. 

Mike Shockley 

Not so, Mike. The following chart is from Shenandoah Div. ETT #5 effective Sunday, May 20, 1973.

Jimmy Lisle

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