Handbook of Railway Vehicle Dynamics

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design layout of the track from the short wavelength features that form the variation from the design
(i.e., the track irregularities). This usually results in adescription based on the following five
geometrical terms:
Curvature —the lateral design layout of the track radii (long wavelength). Curvature is defined
as the inverse of the curve radius in units ofrad/km. However curvature may also be quoted as
a“versine” measurement in mm, this being the distance from the centre of achord of known
length to the rail.
Cant —the vertical difference in height of the left and right rails, (long wavelength).
Lateral alignment —the short wavelength lateral track irregularities.
Vertical alignment —the short wavelength vertical track irregularities.
Gauge —the distancebetweenthe rails measured at aspecified distancebelowthe crown of the
rail. This is typically 14mminthe U.K. and Europe and 5/8 in. in North America. Gauge may
be given as an absolute value oravariation from anominal gauge (e.g., 1435mmEuropean
standard gauge).
The geometry of railway track may be recorded using one of several techniques.
A. M ANUAL S URVEY
The surveyor establishes adatum (or several datums)position on the site to be surveyed, usually by
placing amarker in the ground.Atheodolite is then used to record the position of the left and right
rail with reference to the datum position. Considerable care is required toproduce accurate results
from these techniques. Good results have been achieved by using ahigh accuracy “autotracking”
theodolite measuring to atarget placed on the fixed end of acant and gauge stick above the rail
gauge corner. The theodolite is then used to measure the position of one rail, and the position of
the adjacent rail is determined from the cant and gauge measurements displayed on the stick
(see Figure 13.24).
In the absence of other methods, useful results may be obtained for track design curvature by
conducting aversine survey. Inthis case, achord (wire) of fixed length is stretched along the high
rail of the curve and the distancebetweenthe centre of the chord and the gauge corner is measured.
Chord lengths of 10, 20, or 30 mare common depending on the curve radii to be measured.
The chord length is normally chosen sothat the measured versine does not exceed 150 mm on the
tightest curve to be surveyed. Successive versine measurements are takenatfrequent intervals, with
the maximum recommended interval being half the chord length. Increasing the measurement
frequency will increase the detail contained in the survey results. The radius of curvature at any
mid-chord position can then be calculated as follows:
R ¼
where: R ¼ curve radius (m); C ¼ chord length (m); and V ¼ versine (mm).
FIGURE 13.24 Cant and gauge stick (courtesy of Abtus Ltd.). Source: Abtus Ltd. —Catalogue. With
permission.
© 2006 by Taylor & Francis Group, LLC
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Handbook of Railway Vehicle Dynamics