Track geometry is three-dimensional path through space of rai tracks. It also describes the measurement data used to assess track condition, so the term is used in design, construction and maintenance of railroad tracks. The subject is used in the context of standards, speed limits and other regulations in the areas of track gauge, alignment, elevation, curvature and track surface.
Measurement and analysis of track geometry is very important to ensure safe operations for vehicles. The most common measurements within track geometry are:
1. Track Gauge
The Track Gauge is the distance between the running edge of left and right rail . Gauge is measured between the running edges of the rail, i.e. a point 14mm down from a line between the two tops of rails. It is not quite the same as the inside edge to inside edge (usually it is slightly more than that)
Standard Track Gauge is 1435mm.
Figure 1: Track gauge measurement
2. Track cant (Super elevation)
The difference between the level of the two rails in a curve is called cant (also called super elevation or crosslevel) and is arranged to compensate part of the lateral acceleration.
Trains operating in curves experience net lateral force (Centrifugal Force) to the outside of the curve that is the function of velocity. With the super elevation (Cant) this centrifugal force acting on the passenger is reduced, or eliminated, by a component of the gravitation force (Weight).
Balance speed (also called equilibrium speed) for any given curve is the speed at which the lateral component of centrifugal force will be exactly compensated or balanced.
The term "Cant deficiency" is defined in the context of travel of a rail vehicle at constant speed on a constant radius curve.
Cant deficiency involves travelling through a curve faster than the balance speed and produces a net lateral force to the outside of the curve. Cant deficiency is measured in inches and is the amount of super elevation that would need to be added to achieve balance speed.
The cant is maximized with respect to stationary conditions and slowly running trains. A maximum value is set for cant because of the following problems which arise if train is forced to stop or run slowly in a curve:
Figure 2: Forces resulting from canted track
3. Horizontal Curve
Whilst radius (“R”) is the most obvious measurement for a curve, it is more common to use curvature ,k=1/R. The radius is related to the center of the track. The vehicle running at a speed v in a curve with a radius R undergoes a centrifugal lateral acceleration:
… which results in many undesirable effects. These effects can be:
Figure 3: The definition of a horizontal circular curve radius R
4. Vertical Curve
Vertical curve is the curve in vertical layout to connect two track gradients together whether it is for changing from an upgrade to a downgrade (summit, a vertical summit is also called a hog), changing from a downgrade to an upgrade (sag or valley), changing in two levels of upgrades or changing in two levels of downgrades. Vertical curve is rate of change of height of track. Short wave vertical (or top) is the local variation of vertical geometry
Typically, maintenance engineers are only interested in local variation in curvature, as the underlying track geometry will already have been designed to cover the underlying hills and valleys along a route.
The distance above track against height diagram is a long section, not a cross section (long section runs along a linear asset, cross sections are at right angles to a linear asset – the diagram with the 2 rails above is a cross section)
Figure 5: Long and short vertical features on a long section
This is not a measure of actual height above sea level but it’s measure of local variation in geometry. So, the above plot is showing distance between a track in x-axis and height above the sea level in y-axis.
We can see in plot that there is a hill, and within that hill we have a local “bump” in the track.
So, what we are trying to do is extract only the information about the localized bumps travelling over the hill. If we get bumps that we can traverse at line speed in less than 2 sec then there is a fault. These types of faults are often cause by poor ballast condition. Deterioration of ballast tends to be very predictable and linear. So, it’s a good thing to run through analysis to predict deterioration.
The topographical conditions usually require vertical-longitudinal gradients, along the way. Building bridges and tunnels is a very expensive way to manage the topography constraints. Heavy railway traffic cannot use track with excessive longitudinal gradients. Therefore, restrictions for gradient are needed. The following requirements need to be considered because they influence railway traffic:
Thus, large gradients result, principally, in heavier locomotives, increased gradient power, and/or less freight train weight, and/or reduced speed and line capacity, and/or requirement of higher braking capacity, and/or larger signaling distances.