Editing Rail stressing (section)

==Background==
Railway steel contracts at low temperatures and expands at high temperatures.<ref name="Bibel">Bibel G. [https://books.google.com/books?id=HiU7KmXmx3oC&dq=expansion+railway+steel+heat&pg=PT249 ''Train Wreck: The Forensics of Rail Disasters''] JHU Press, 2012 {{ISBN|1421406527}}</ref> In extreme cold, a length of CWR suffers [[tensile stress]]. This stress can cause railway steel to fracture.<ref>Brockbank W. [https://books.google.com/books?id=IgVCAQAAIAAJ&dq=fracture+steel+railway+extreme+cold+tensile+stress&pg=PA76 ''The Effects of Cold on Iron and Steel''] The Railway Times vol 23 p76 11 March 1871</ref> In extreme heat, a length of CWR suffers [[compressive stress]]. This type of stress can cause [[Buckling#Rail_tracks|sun kink]] where a length of a railway buckles laterally (sideways).<ref>{{cite book |last1=Li |first1=Dingquing |last2=Hyslip |first2=James |last3=Sussmann |first3=Ted |last4=Chrismer |first4=Steven |url=https://books.google.com/books?id=L92YCgAAQBAJ&dq=heat+compressive+stress+railway+buckle&pg=PA430 |title=Railway Geotechnics |publisher=CRC Press |date=2002 |page=430-431 |isbn=148228880X}}</ref> Other factors that affect CWR condition include the state of the [[track ballast]] and its shoulders; the type and placement of the [[Railroad tie|sleepers]]; places of increased shadow such as tunnels and bridges; track consolidation; and, to a lesser degree, the vertical curvature of the tracks.<ref>[https://books.google.com/books?id=F5s8AAAAIAAJ&dq=CWR+railway+condition&pg=PA91 ''The Code of Federal Regulations of the United States of America''] U.S. Government Printing Office, 2000 p91.</ref>

In the design and installation of CWR, a figure known as the "rail neutral temperature" (RNT) is calculated. The tensile and compressive longitudinal forces on the CWR are added. The RNT is the notional temperature when the total equals zero.<ref>Pyrgidis C. [https://books.google.com/books?id=e5ymCwAAQBAJ&dq=%27rail+neutral+temperature%22&pg=PA66 ''Railway Transportation Systems: Design, Construction and Operation''] CRC Press, 2016 {{ISBN|1482262169}}</ref> A similar figure is the "stress free temperature" (SFT). It is the rail temperature at which the rail is the same length as in its unrestrained state.<ref>Chang F. [https://books.google.com/books?id=RPXU1vBhIFMC&dq=CWR+stress+free+temperature&pg=PA1314 ''Structural Health Monitoring 2003: From Diagnostics & Prognostics''] DEStech Publications, Inc, 2003 p1314. {{ISBN|1932078207}}</ref>