The renaissance of rail travel in many parts of the world has led to both requirements and opportunities for optical fibre deployments. In the UK alone, rail journeys are up nearly 70% compared with 2003. New rolling stock is rapidly being introduced, lines renovated and even new stations opened; in the case of the UK the average age of the rolling stock is 20 years so the new vehicles will be noticeably more advanced. The same picture applies across the European Union with Germany the single largest rail user. However, the number of annual fatalities is worryingly increasing Europe-wide, despite this form of travel being the safest mode around.
Fibre for signalling
Historically signalling failure have been a major factor in both safety and service delay incidents. Naturally when the signal itself fails or the link to it goes down the signal defaults to a ‘stop’ condition. Copper cable connections between signals and especially the closures associated with them, particularly when frequently re-entered are prone to water ingress, corrosion and short-circuiting. Properly designed and installed fibre links mitigate this problem substantially. A recent example is shown here:
Monitoring the Rail Network
As train numbers and the weight of trains increases the load on the network – rails, bridges and other structures – has become significantly greater. For this reason, a great deal of work has gone into fibre sensing systems. One of the most important devices is the Fibre Bragg Grating (FBG) which is a very small (typically 1-5mm long) section of optical fibre which has been ‘sensitised’ by having the core’s refractive index modified periodically. The modification is done by the sensor maker, using a UV light source and a pattern or template applied to the fibre. The FBG reflects a specific wavelength which changes both according to the strain imposed on the fibre and the ambient temperature. In each case the device has excellent sensitivity and came be made part of the cable without needing to splice it in place.
The variety of applications is widespread. In Hong Kong a holistic system has been deployed where sensors are fitted to the track rails to measure distortion and to the rolling stock to record the loading of carriages and the temperature of the brakes and other critical systems.
It’s perhaps worth taking a little time to look at one specific application; the so called ‘fishplate’ evaluation. Fishplates are the devices that join two rail sections together via bolts and parallel plates. Some sections of track place higher loads on the fishplates, for instance the exit/entry to sharp curves. In these cases, FBGs bonded to the fishplates in multiple axes can help determine instances of loosening bolts, forestalling the possibility of an accident.
Fibre On the Train
As well as carriage load monitoring, fibre on trains is becoming useful for communications systems both for staff and for public use (wi-fi). Indeed, a new 100 Gigabit-Ethernet Standard has been developed that can be used on trains. Although a copper based cable system could be used, the immunity of a fibre based system to the high voltage environment of a rail system is very attractive. However, fire performance on rail networks is of vital importance and in Europe the various parts of EN 45545 are being introduced to meet the objectives of EU Directive 2008/57/EC.
An overlooked topic in the rail environment concerns optical connectors which can be subjected to vibration and general loading considerably in excess of that usually found in telecommunications networks. In the absence of fully developed standards in this field it is wise to consult industry experts on this topic.
Deploying a Network in the Rail Environment
As well as the challenges outlined above, the rail system offers some distinct advantages for the installation of fibre cable. First, rail routes are nearly always level and use bends that are controlled and, above all, the entire route is well documented. This means that for cable puller or blower it is relatively easy to work out cable spans and site joint boxes and other interconnects. Secondly, access to the network is very controlled and limited in duration; since most telecoms faults occur after network intervention this helps cut fault rates. Moreover, those personnel that do intervene in the network are nearly always highly trained and experienced.
Thirdly, with the exception of level crossings, the railway has primacy in its right of way and is not generally bisected by other services. Ironically within otherwise crowded shared city infrastructures, the rail network sits on high value, protected and very useable real estate.
So the needs of the rail operators and envious telecoms providers have converged. This means there is a plethora of different services needing space on the railway: signalling cables, monitoring cables, camera control cables, railway voice communication cables as well as – potentially – the cable requirements of one or more 3rd party dark fibre or managed service telcos. The challenge here is that although these cables need a common path along the railway they do not necessarily originate of terminate at the same destinations.
One attractive option is to use multiple microduct packages that can accommodate several different cables. The individual microducts can be broken out and the specific cables routed to their individual destinations. Trackside there are several options for deploying – in concrete troughs with or without covers, trenched into the ground or even mounted aerially where an overhead power feed exists. It is also possible to fix cables to the foot of individual rails. These solutions require various degrees of protection for the ducts and cables. The concrete trough solution is one of the oldest but puts surprising demands on the cables. The weight of cables successively added can mean those at the bottom of the trough are difficult to access and the trough itself can become very hot or very cold leading to expansion/contraction issues that are not seen with buried cables. So looking to the future, solutions that avoid these troughs may become prevalent.
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