On Tue, 5 Aug 2003 07:03:45 +0100 Clive D. W. Feather wrote:
}
} In this case we're talking about basic physics. When you heat metal, it
} expands. When you heat rails, they get bigger. That expansion has to go
} somewhere.
}
} Now back in the days of jointed rails you could put gaps between the
} rails for them to expand into, but even then eventually the gaps filled.
} With modern welded rail (providing a much smoother ride) you can't have
} gaps. So, you anchor the rails firmly and install them so that they will
} be at the right length when the temperature is 25 degrees C. If it gets
} colder, they contract but the anchors pull on them, so the contraction
} puts the rail under tension. If it gets hotter, they expand but again
} the anchors pull on them and the rail goes under compression. Note that
} the hotter it is, the more compression they are under.
}
} If the rail is severely compressed *and* receives a severe blow at the
} wrong point, the clips holding it on the sleepers and in line might not
} be able to take the full force without giving way. Now the rail comes
} free of the clip and buckles. This is clearly dangerous. So one thing
} that can be done is to reduce the chances of a severe-enough blow, and
} one way to do this is to reduce the speed of trains (track forces
} increase with speed), particularly in areas at greatest risk.

This morning's Radio 4 news had it that "trains normally running at 110
mph" would be restricted to 60 mph.

Will such speed restrictions really make a significant difference in the
likelyhood of an incident such as you describe or is it aimed at
reducing the damage in such an event?

Tangentially why should this happen now and not in past years? After
all it's far from being the first time we've had a remarkably hot spell.

Matthew
--
Il est important d'être un homme ou une femme en colère; le jour où nous
quitte la colère, ou le désir, c'est cuit. - Barbara

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