The running of the trains over the railway track is inducing vertical and lateral forces that, in time, are affecting the track alignment, changing incrementally the installed track and causing track irregularities.  On curved track this deterioration process is more significant due to the additional effect of the lateral (centrifugal) acceleration. When the irregularities become isolated track defects require punctual maintenance actions to correct them. When there are multiple track defects, or even when the track irregularities are significant and affect the riding quality, the required maintenance remedial actions are more complex and focused on the rectification of the track geometry. To do this the track needs to be surveyed and, based on this survey, a design is produced to adjust and correct the existing geometry.

Track surveying

There are essentially two ways of setting about the realigning an existing railway (Cope, 1993). In the first method the track is surveyed using topographical optical equipment. The collected data is an accurate image of the existing track layout and is used as reference for the definition of a design geometry composed of straights, circular curves and transition curves. This is currently done using specialised design software which uses the survey data to run for finite sections of track and potentially individual alignment elements a regression analysis which uses least squares or other regression methods to optimise the design and propose alignment adjustments that requires minimum mechanical work. This complex design process is required for major track works.

For the second method the railway track is surveyed by taking offsets from a chord to the rail running edge. The measured chord offsets are called versines.

Versine surveying (source of the image: www.aldonco.com)

This measurement is done at the centres of successive overlapping chords laid out along the outer or high rail of the track.  This chord overlapping is essential to define the existing track alignment:

There is a direct relation between the versine and the track curvature (see The Versine Formulae). These versines are used in another type of track correction design called string lining realignment or rectification. This design consists of determining the lateral adjustment (slews or slues) required to correct the track and smooth the versines as close as possible to an ideal set of values.

The entire process became known as stringlining or the Hallade method. It comprises of two main parts: the versine measurements, called Hallade survey and the design process of computing the slews from the measured versines, sometimes called Hallade design. This is an acknowledgement of the inventor of the method, the French (Swiss?) engineer Émile Hallade.

Track Realignment and Rectification

These “Hallade” design methods used for track alignment correction can be divided in two categories (Radu, 2003):

• Track Rectification – used for local adjustments of the track but can be extended on an entire curve, without changing significantly the length and curvature of the alignment elements. In this case the design versines are usually known and used as the target results for the computation process.  This category of methods is based on proposing individual or successive slews and evaluates their effect on the versine variation.
• Track Realignment – a complex procedure which is affecting an entire curve or set of neighbour curves, changing the length and curvature of the alignment elements. The realignment produces a new track alignment and can be used to improve the track geometry to allow speed increase.

For both set of methods the design is satisfactory if it provides a set of proposed versines within the design tolerances to the ideal, nondeformed track geometry.

References:

• Cope, G. (1993) British Railway Track – Design, Construction and Maintenance. Permanent Way Institution, Echo Press, Loughborough.
• Radu, C. (2003) Rectificarea si retrasarea curbelor de cale ferata (Rectification and realignment of the railway track curves). Course notes. Technical University of Civil Engineering Bucharest.