When designing multiple line alignments – in railway stations or plain line sections – there will be a creep of the alignment stationing due to the length difference between the alignments.
This stationing (or chainage) creep becomes relevant when adjacent alignments have to be kept at the same level or with a well-defined elevation difference. This might happen at platforms, level crossings, S&C layouts and structures with controlled or limited clearance.
The chainage creep is caused by:
- Diverging alignments. If the tracks are not parallel the projection of one over the other does not produce the same length. L1 < L2
- Siding lines. Usually the siding line of a turnout has a different (longer) path compared to the main line. This is particularly important when that siding is required to be connected back to the main line, through a turnout.
- Presence of curves on parallel alignments. This is the most common cause of chainage creep. Parallel curves have different lengths and this difference is dependent on the curve length and radius.
For an UK conventional track interval (TI) of 3.405 m and for parallel alignments, the chainage creep due to a 1000 m long curve and different radii is summarised in the following table:
The chainage creep main influence is on the level difference between adjacent tracks. This is highlighted in the table by comparing what is the gradient G influence on this level difference. If the vertical element is steep enough then the two alignments would have, for the same gradient, different elevations and this difference will increase along the curve and be present on the following horizontal elements. For example, two parallel 1 km long curved alignments – of 1000 m radius – for a 1% gradient on both will have at the end of the 1 km section a level difference of 34 mm. If that gradient is 0.1% the diffeence would be only 3.4 mm – which can be ignored for most of the cases but, nevertheless, aknowledged.
The easiest to overcome this issue is to define a different gradient for each line, defined by the equation:
G1 L1 = G2 L2
In this case, even if the two gradients, G1 and G2, are different, the elevation of both alignments along that curve would be identical. In a way, this is similar with the use of different rail lengths on jointed curved track to keep the joints at square.
If the gradient is extended on adjacent horizontal alignment elements this simple approach cannot be applied and the gradients are required to be correlated dependant on the point where the elevation of the two tracks is relevant to be aligned.
Most of the modern alignment design software are able to project the stationing of an alignment on any other and to present an anamorphic stationed vertical profile. This allows the designer to easily develop or review the vertical alignment for multiple railway lines and takes out the effect of the chainage creep.
Another way of dealing with this is to change locally the alignment stationing reference inserting Special Stations or Station Equations. This change of the station reference allows the user to control better all the issues generated by the chaiange creep.