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Downeaster delays due to heat
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montana mike
Conductor
Just curious--Home come in the northern tier of the US the trains are slowed down due to "heat" at 85 degrees or higher in the summer (a frequent problem on the Empire Builder thru MT and ND in the summer), yet one hardly hears about "heat" issues throughout the south and southwest? I just came back from a trek on the CONO (right on schedule for both legs of my trip)-with temps in the upper 80's and low 90's during parts of both segments, and we were trekking along at normal track speed during this time thru LA and MS.
Ryan
Court Jester
It has to do with the temperature differentials and the neutral temperature that the rail is laid at.
Down south, they don't get the subzero temps that ME does.
This article has some basics:
http://en.wikipedia.org/wiki/Rail_stressing
Down south, they don't get the subzero temps that ME does.
This article has some basics:
http://en.wikipedia.org/wiki/Rail_stressing
crescent2
Conductor
Very interesting, thanks, Ryan!
Ryan
Court Jester
No problem - now that I'm my computer and not the iPad, I can explain a little better.
When rail is laid, there is a temperature at which the rail isn't under any stress - it isn't under tension, nor is it being compressed. From that state, if it heats up too much, the rail will want to expand (hot rail gets longer), but can't because of the fixed ends. Eventually, if you keep heating up, you get a "sun kink" where the force is great enough to push the rail sideways out of alignment.
Going the other way, when it gets cold, the rail wants to contract - as it gets colder, the tension in the rail will increase, until it's sufficient to crack the rail.
Generally speaking, cracked rails are "better", as they'll show up in the signal system and trains will get a stop signal. Sun kinks are a different story - because the rail stays intact, it won't show up in the signal system, and the first warning you'll get is when a train sees it - maybe it'll be able to stop in time, maybe not.
With that in mind, down south the rail can tolerate hot weather because it's laid using a higher neutral temperature. If rail were laid in Maine at that same temperature, you'd have cracks galore come wintertime...
When rail is laid, there is a temperature at which the rail isn't under any stress - it isn't under tension, nor is it being compressed. From that state, if it heats up too much, the rail will want to expand (hot rail gets longer), but can't because of the fixed ends. Eventually, if you keep heating up, you get a "sun kink" where the force is great enough to push the rail sideways out of alignment.
Going the other way, when it gets cold, the rail wants to contract - as it gets colder, the tension in the rail will increase, until it's sufficient to crack the rail.
Generally speaking, cracked rails are "better", as they'll show up in the signal system and trains will get a stop signal. Sun kinks are a different story - because the rail stays intact, it won't show up in the signal system, and the first warning you'll get is when a train sees it - maybe it'll be able to stop in time, maybe not.
With that in mind, down south the rail can tolerate hot weather because it's laid using a higher neutral temperature. If rail were laid in Maine at that same temperature, you'd have cracks galore come wintertime...
95 degrees is the typical temperature above which the CZ begins to encounter speed restrictions in western Colorado and Utah. I have experienced this a few times, and the dispatcher will typically reduce maximum speed limits to 50 mph from the normal 70 or 79 mph.
George Harris
Engineer
ZST = zero stress temperature is that temperature
There is a formula used to decide what it should be for a given location. (2H+L)/3+C. That is, twice the highest temperature the rail gets to in a year plus the lowest temperature the rail gets to in a year. Add these, divide by three. Add some number of degrees that seem reasonable, usually something on the order of 15 to 25 degrees. Most railroad companies have a number they use, or a map showing what temperature to use where. Depending upon what part of the country, this ZST will be somewhere between around 85 degrees and 120 degrees. As Ryan says, in Maine is will be less than it would be in places like Arizona and New Mexico, or a lot of other places, either. The general idea in the temperature selection is to keep the rail in tension most of the time. Emphasis on rail high temperature because the sun can heat the rail to up to 30 degrees hotter than the air temperature.
Ryan
Court Jester
Thanks, George.
If you really want to dig into this (the general "you", not George who lives and breathes this stuff), this NTSB report is excellent reading:
http://www.ntsb.gov/doclib/reports/2003/RAR0302.pdf
It's about the Auto Train derailment about 10 years ago that happened when it ran over a sun kink and gets into a great amount of detail (but explains it very well).
If you really want to dig into this (the general "you", not George who lives and breathes this stuff), this NTSB report is excellent reading:
http://www.ntsb.gov/doclib/reports/2003/RAR0302.pdf
It's about the Auto Train derailment about 10 years ago that happened when it ran over a sun kink and gets into a great amount of detail (but explains it very well).
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The .pdf makes a worthwhile read - thanks for posting the link.
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