377 on Thameslink
wrote:
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No, because the wheels are not a fixed diameter, but get gradually
smaller as they are turned on lathes to remove flats and
imperfections. A small difference in diameter will lead to a large
cummulative error, if no corrections are made. There are other systems
which measure distance travelled (for example radar), but I don't know
how they are affected by different track formations etc.
I thought I heard that some lines work on revolutions, particularly the
Central Line.
If this is not the case, then how do Central line trains know where they
are?
They ask Underground Ernie, and he tells them?
;-)
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