Tonnage reduction with temperature falling

NW Mailing List nw-mailing-list at
Mon Dec 17 10:39:06 EST 2018

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> 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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