What is Track Engineering?
So you have read about the railway system, with all its different parts and disciplines. Now to get some more detail on track engineering.
Track engineering is a wide discipline that covers a lot of topics. It involves all stages of the lifecycle of the track, from its initial design through to its maintenance and finally its redesign and renewal. It also includes disciplines related to the design of specific components
Track Bed
As mentioned in the railway system introduction, the foundations on which railway components are laid is key. It must be stiff and strong enough to withstand years of high loading from train traffic. It also has to be able to withstand environmental factors, such as restraining thermal expansion in warm weather, drain water away in heavy rain etc.
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It also has to be maintainable as the asset ages, as well as allow other track components to be changed or maintained. Failure of the track bed can lead further wear and damage to other componentry as well as track geometry faults that pose a risk of derailment to trains. Ballast can be dug out and replaced, it allows the track to be lifted and slued all the while giving the track the support it needs.
Track Components
The key components that make up the track (sleepers, baseplates, fastening and rail) have all evolved over time. This has led to a varied of types, all of which track engineers have to be aware of.
Selecting the correct type of component, such as rail section or sleeper type, can have a long last impact on the asset, its life and maintenance requirements. Railway standards also govern how components are designed and employed with the railway, and adherence to these standards is important to ensure the safety of passengers and trains.
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As the complexity of these components, along with the engineering behind them, has increased specialisms have developed and become move imbedded within the railway industry. Metallurgists look at the composition of the steels used for rails, switches and crossing to reduce defects and wear. Extensive research is undertaken on the detailed design behind switches and crossing such as analysing common failure mechanisms, how train wheels interact with the switch during transit across it, all with a view to further improve safety and the design itself.
Track Layouts
Track can be categorised along two types, plain line and switches and crossing (S&C). Plain line makes up the majority of the railway, where the track is of a simple construction of two rails on sleepers. Switches and crossing layouts are used at junctions to allow trains to be moved across to different lines. These can range from simple crossover layouts to complex layout, such as those in large stations.
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These two areas of track have different design values and limits that govern them, along with different engineering considerations. S&C also brings key interfaces with other disciplines as well as the operational side of the railway. They are a critical part of the railway’s operation, where faults or issues can cause major disruption.
Track Geometry
A railway line can be considered as straight sections joined by curves between its origin and destination. This holds true for the horizontal (left and right curves) and vertical (up and down gradients and curves). The designing, construction, measuring or maintenance of these elements is referred to as track geometry.
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Normally it is separated quite definitively into horizontal and vertical plains. The design of the track geometry is linked to the speed the trains are safely able to travel at, as well as the forces acting on their cargo. An important factor when designing specifically horizontal geometry is these forces and their impact on the comfort of the passengers travelling on the train.
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Other important factor that track geometry can influence is the rail wheel interface. A train wheel and the rail head are, in the most basic form, two metal services interacting. They both wear and can suffer damage. Rolling contact fatigue and side wear are common issues that arise with rails that are caused by the wheel rail interaction. Good geometry design can help reduce the damage to both rails and train wheels.
Gauging
Structures such as tunnels, bridges and platforms as well as railway equipment including signals and speed signs are built close to the track on which the trains run. Gauging is the process used by engineers to ensure that trains don’t strike these pieces of infrastructure, or indeed each other as they pass.
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There are a number of sizes and shapes of trains that run on railway, each with its own characteristics in terms of the extents and way it moves while travelling. The trains carriage or freight wagons center of gravity, its dimensions in terms of length and width, where its wheel sets sit in relation to each other, and the ends of the carriage along with the suspension stiffness are a few of the factors that influence how the different trains move while in motion. ‘Swept volumes’ are used to represent the maximum extents at which a type of train moves, giving a bubble in which, no items of infrastructure can be without being hit, or ‘foul’. Swept volume templates are used in software to analyse proposed designs to check the clearances to structures and by maintenance to monitor existing clearance for deterioration.
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The piece of infrastructure most familiar to the general public will be the station platform. It is the key area where passengers and trains interact, with people boarding and leaving trains. The distance of the step between the train doors and the platform, known intuitively as stepping distance, is another area looked at during gauging. This is to avoid large gaps horizontally between the train and platforms that passengers could fall down, or large steps up/down vertically that could cause less mobile passenger’s issues. Providing good stepping distances for a variety of passenger train type while also obtaining good clearance values for all rolling stock (passenger and freight classes) can be a troublesome exercise for track engineers.
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Track Maintenance
To ensure that as much of the designed life of the asset is realised as possible, regular maintenance and inspection needs to be undertaken. This ensures that the railways are safe for the running of trains, reduces faults that cause disruption and prevents deterioration of the assets making up the railway.
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A key part of maintenance is inspections. They are carried out visually by staff members, via train borne inspection equipment or other testing equipment. These inspections, carried out at intervals defined in a relevant standard, can identify faults and defects that require rectification.
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Within Track, the most prevalent inspection undertaken is traditionally basic visual inspection. This inspection involves staff walking all the sections of track, at set frequencies which are based on the track type and its frequency of use. Track inspectors form both the first line of defence, by spotting issues early and monitoring their deterioration, but also the last line as they able to react quickly to protect train safety when a major fault is discovered.
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This type of inspection is being replaced by the use of train borne inspection technology called Plain Line Pattern Technology (PLPR). This removes the need for inspectors to walk mile and miles of track, often while trains are still running. The PLPR trains also monitor track geometry; top, line and gauge. Where dips and other geometry faults are recorded, these are raised with a rectification timeframe for the maintenance teams to fix.
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Train borne inspection is becoming more prevalent as a way to reduce staff having to step onto track, a key way to improve safety. The number of types of these inspections is increase, such as UTU trains scanning rails for internal defect, gauging trains monitoring the distance to lineside equipment and other trains inspecting overhead lines.
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Detailed inspections are undertaken on specific part of the track, such as longitudinal timber bridges or switches. These require ta more specific inspection due to the higher risk nature of these asset types.
Rectification of the faults and defects found by the inspections is the other main maintenance activity. Rail defects are removed by localised weld repairs or replacement of the rail, switch or crossing. Defective components, such as a rotten sleeper, spotted by the patrolman or the PLPR train, are replaced. Track geometry faults are rectified either manually, most likely for smaller faults such as dips and twists, or for larger faults such as cyclic top mechanical means are used. Manually involves the use of jacks to lift and correct the position of the track. Engineering trains called tampers and stone blowers are the mechanical means of removing track geometry issues.