HomePermanent WayThe trackside conduit

The trackside conduit

Data connectivity is vital for all aspects of society and industry, and is becoming increasingly important for railway operations. Signalling, electrification control, fixed and radio communications: all rely on cables running alongside the railway. However, the integrity and value of these cables is only as good as the protection offered by the lineside cable route. Often overlooked or taken for granted, it is vital for the safe, efficient operation of any railway.

The primary function of any form of cable route is to protect the cables within. Ideally it should provide mechanical protection, so it must be robust and stable, fire and vandal proof, capable of being opened and closed to maintain the cables or run new ones, be cost effective and safe to install, and require minimal maintenance. Cable routes can be constructed from a variety of materials and come in different sizes and shapes. Transition modules may be required along with ‘T’ junctions, bends and joint bays.

All these requirements are not easy to achieve and can conflict. An assortment of route types have been used over the years and the search for an ideal cable route is never ending.

An evolutionary approach

The first types of cable routes were open copper wires raised on wooden poles. With the introduction of overhead electrification, signalling and telecoms cables were run at ground level in some form of cable containment. On the majority of non-electrified lines or those with conductor rails, cables are now provided at ground or sub-surface level. Wooden cable routes were used initially, but these soon rotted and have not been used for many years. Asbestos cable routes mounted on posts were then provided, but these didn’t offer much mechanical protection and introduced a health and safety risk.

One of the more successful systems was concrete ground-level troughing, generally known as GLT. With detachable lids, this has been used for many years and is supplied in various sizes, generally in 1m lengths. It provides reasonable mechanical strength, although the natural walkway it forms encourages its use as a lineside pathway. It was not designed for this purpose and a misaligned lid can easily cause injury if walked on.

The weight of concrete troughing gives it reasonable stability, but makes it difficult to handle manually and its installation can require extensive possessions for off-loading. Deeper and higher ballast shoulders have, in places, transformed the troughing into ballast retaining walls or totally burying it. Any displaced alignment may put strain on the cables within and make it even more hazardous to walk on. While concrete troughing is reasonably inexpensive for the protection achieved, it is costly to install with manual labour and requires frequent replacement of damaged lids.

In the 1960s, to obviate many of the disadvantages of the precast concrete route whilst retaining its inherent advantages, a continuous slip-formed concrete route was trialled. A train with an earth plough formed a trench alongside the track. Concrete was then discharged from the train into the trench, after which the train made a further pass with a plough to create a trough in the concrete. Once dry, precast concrete lids were off-loaded.

This method removed much of the manual labour involved in construction, but required a site with soil suitable for the earth plough, precise control of the concrete mix consistency, accurate rate of discharge into the trench and consistent train speed. It was also found that an unplanned thunderstorm turned a near-finished length of route into a disaster area! The trial was abandoned.

Buried cables

In areas of theft risk, cables can be buried directly in the ground at a depth providing adequate protection. Installation is generally required immediately after the trench has been dug as heavy rain can collapse it prior to backfilling. Alternatively, a duct system can be buried prior to the cables being pulled through or, in the case of fibre cables, blown through with compressed air.

Any buried route is expensive to install – particularly with manual labour – and may require rail-mounted machinery for excavation. Gaining access to the cables requires careful selection of the breakout point locations, together with jointing bays. The chosen cable must be suitable for direct burial and it is not easy to subsequently connect into the cable route.

Another method of direct burial is by utilising a rail or vehicle-mounted mole plough. This is a hydraulically controlled plough blade, with the cables fed through conduits in the blade. In the right situation the method has some advantages, but large cables can present problems as certain types of soil do not consolidate and can leave a damaged embankment or cess. The method requires careful planning and preparation, with robust buried service checks prior to the cable-laying. It only takes one old signal base to break the plough and seriously delay the programme. In locations with sharp stones, additional cable protection is required with sheathing or sand backfill. However, ploughing small fibre cable ducts could be an answer for the future. We’ll come back to that later.

Plastic routes

Over the years, various sorts of plastic cable routes have been trialled for lineside cable containment. Unfortunately, they have generally not provided the required mechanical strength, especially when faced with ballast alongside the route. Plastic also contracts and expands as the temperature varies and it is not easy to incorporate adequate expansion mechanisms.

Railtrack had some success installing a 100mm plastic surface pipe with thermal expansion mitigation which was staked into the ground every few metres. This provided protection to a telecoms cable running between signal boxes on rural routes and is still in use 25 years later. This wasn’t suitable for larger volumes of cables on main lines and it wasn’t easy to provide regular cable breakout points.

Recycled polymer

Over the last ten years, cable routes made from 100% recycled polymers – such as polypropylene – have been introduced which offer similar high strength and impact resistance to traditional concrete, but are approximately five times lighter for the same size and far easier to cut. A further enhancement – building on the problem of walking on narrow concrete lids – was the introduction of a combined cable route and safe walkway, also made from recycled polymer. Two routes with the equivalent capacity of two concrete troughs were located under a 700mm wide non-slip surface. The lids were constructed so that cables could be laid with half the route open. Fixings were also provided for a removable handrail and the ability to secure the lids to the troughing to deter cable theft.

Following removal of the lids for installation purposes, there have been reports of a route’s sidewalls being deflected inwards due to the weight of the adjacent ballast, preventing the lids being replaced correctly. This caused the lids’ outer edges to be unsupported and move unexpectedly under the weight of footfall, thus creating a trip hazard. This illustrates the need for maintenance of all cable routes and, in this case, to ensure the lids were securely attached to the sidewalls and not displaced from their correct positions.

Elevated troughing

In cuttings susceptible to slippage, the toe cannot be excavated to accommodate a trough or buried route, so an elevated troughing route may be required, mounted on posts which only interfere with the soil formation at a minimal number of points. As well as early use of asbestos, elevated troughing has been constructed from a variety of materials over the years including timber, cement, glass-reinforced plastic, metal, recycled polymer and glass-fibre reinforced concrete (GRC).

Many elevated routes lose their alignment during their lifetime due to movement in the soil foundation and are particularly prone to damage as they form an obstruction to track work and make natural seats. Nevertheless, elevated GRC routes have, in particular, been widely used and are easy to transport and install.

Theft and vandalism

Vandalism and theft of copper cables have produced particular problems. Concrete routes may require lids to be fixed with metal clips or epoxy adhesive to deter theft. In high-risk areas, cables have been sealed into GLT with concrete, but this is generally not recommended as cement can harm some cable sheath types and filling the trough route with concrete makes it unusable for other cables. It is also expensive and labour intensive. Putting blobs of concrete in the trough is not good either as thieves have been known to cut and steal the intervening length, making it difficult to run a replacement cable.

Buried routes are quite effective as a deterrent if sufficient depth is maintained and the soil is well consolidated, although it has proved necessary in some locations to anchor the cables to prevent them being pulled from the ground using road vehicles.

Fibre cables

When the national fixed telecoms network was deployed by Network Rail, a heavily armoured variant of the normal armoured optical fibre was chosen to deploy without any cable route. This was known as Double-Insulated Super Armoured Cable or DI-SAC which was approved for use where only optical fibre cables were required.

DI-SAC comprised 24 single-mode optical fibres divided equally into two stainless steel tubes, helically wound around a solid aluminium former, encased inside a medium-density polyethylene inner sheath and thick steel-wire armour, with a green over-sheath of fire-retardant ethylene vinyl acetate.

In walking areas and those prone to vandalism, DI-SAC was scratch-buried so it did not protrude above ground level. Nominally, it was secured into the ground every 40m, but this distance varied to prevent the DI-SAC being pulled onto the track. Its use without a cable route was questioned by many in the industry, but it saved the national fibre project several hundred million pounds. The cable was specially made in high volumes and is no longer commercially available.

Blown fibre

Fibre optic technology has now become the norm for telecoms transmission as it provides huge data transmission capability, solves the problem of inductive interference with long distance copper cables and has no theft value.

Fibre cables were traditionally installed in concrete trough routes in lengths of up to 2km. Selected fibres would be broken out to connect to the digital transmission equipment. Originally, such equipment would use local copper cable tails to provide connections to equipment such as telephones, data terminals, radio base stations and signalling interlockings. With the introduction of all-IP (internet protocol) networks, fibre-borne digital signals right to the end device are now becoming the norm which has led to the concept of ‘blown fibre’ as an option.

This involves a composite material pipe incorporating several ducts of different sizes being installed either on the surface or buried. Bundles of fibres can then be blown into the duct using compressed air and further bundles can be similarly installed into different tubes at a later date, as required.

One drawback of normal routes is that new cables tend to be laid on top of existing cables in the route. When new signalling or telecom systems are brought into use, the redundant cabling remains in place and the trough becomes over-full. With fibre blowing it is relatively easy to remove fibre bundles and replace them should this be required.

Conventional troughing routes and copper cables are far larger than the blown-fibre solution and more expensive to install. As the blown-fibre duct can be coiled and directly ploughed into the ground using smaller machinery, it may be more cost effective and flexible than traditional buried routes. Blown fibre has been trialled in Scotland for lineside installation, but is yet to receive national approval due to concerns with route expansion. At the very least, blown fibre may be a good option for fibre in buildings and stations.

So, the search for the ideal cable route continues in order to maintain the integrity and value of the vital cables which support a safe, efficient railway.

Paul Darlington CEng FIET FIRSEhttp://therailengineer.com

SPECIALIST AREAS
Signalling and telecommunications, cyber security, level crossings


Paul Darlington joined British Rail as a trainee telecoms technician in September 1975. He became an instructor in telecommunications and moved to the telecoms project office in Birmingham, where he was involved in designing customer information systems and radio schemes. By the time of privatisation, he was a project engineer with BR Telecommunications Ltd, responsible for the implementation of telecommunication schemes included Merseyrail IECC resignalling.

With the inception of Railtrack, Paul moved to Manchester as the telecoms engineer for the North West. He was, for a time, the engineering manager responsible for coordinating all the multi-functional engineering disciplines in the North West Zone.

His next role was head of telecommunications for Network Rail in London, where the foundations for Network Rail Telecoms and the IP network now known as FTNx were put in place. He then moved back to Manchester as the signalling route asset manager for LNW North and led the control period 5 signalling renewals planning. He also continued as chair of the safety review panel for the national GSM-R programme.

After a 37-year career in the rail industry, Paul retired in October 2012 and, as well as writing for Rail Engineer, is the managing editor of IRSE News.

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