The installation of ATP elements and the separation of light rail services from interactions with other road users in Stuttgart has helped to reduce accidents and increase service efficiency.
Tramways and light rail systems are generally considered among the safest methods of transport, carrying billions of passengers every year with relatively low incident rates. This is despite potentially facing greater risks and challenges than heavy rail networks through their interaction with public and other vehicles, for example.
However, a number of major incidents have brought safety to the fore in recent years, headlined by the Sandilands (Croydon, UK) tragedy in 2016 that saw seven people killed and 62 injured. It was claimed to be the “deadliest tram accident in the UK since 1917”; a number of recommendations have since been made and already implemented.
The UK is not alone in its continuing efforts to keep light rail at the top of safety records, nor is it the first country to have faced tragic accidents. The German city of Stuttgart has faced similar situations and has instigated a range of improvements that perhaps the UK and others can learn from.
Stuttgart’s Stadtbahn dates back to 1868. Today, the city’s transport network consists of light rail, bus routes and a funicular railway. Its light rail system covers approximately 130.5km (80 miles) across 18 lines and is continuously extending, the most recent project being improved tunnel access in the city centre to allow easier transfers between light and heavy rail stations. The network covers much of Stuttgart, sprawling out to the neighbouring towns of Remseck am Neckar, Fellbach, Ostfildern, Leinfelden-Echterdingen and Gerlingen. It carried 180m passengers in 2017.
It is operated by Stuttgarter Straßenbahnen AG (SSB), the result of tram company mergers in the early years, and uses 204 LRVs as well as 264 buses and one funicular, a rack tramway line, plus a heritage tramway and a steam-powered miniature railway at Killesberg Park.
Light rail has seen a series of changes over the years, with conversion to electric traction in 1895, the introduction of tunnel sections in the 1960s (first to metro standards, then to tram operation), switching from metre- to standard-gauge in the 1980s (all to heavy metro standards) with the first standard-gauge lines opening in 1985. Since then, the lines have been upgraded, with the latest works finishing in 2007 and lines continuously being extended further. The current network is all standard-gauge, except for the heritage line which uses a third metre-gauge rail.
A series of unfortunate events
One major accident took place on 8 December 1970, while the SSB network was still operating on metre-gauge infrastructure and running on line-of-sight principles; at the time, the 1960s-built tunnels were not yet equipped with a train protection system. One tram was descending a ramp into our first tunnel station, negotiating a right-hand curve before stopping at the platform. It was a double platform, and the approaching tram crashed into the waiting vehicle.
The subsequent investigation resulted in a number of improvements, including the fitting of a metro-style Automatic Train Protection (ATP) system in the underground sections. Five years later, this had been installed. However, just weeks before approval was granted for the new system’s use there was another serious accident. On 14 August 1975, a tram began its approach to a 1km straight tunnel at the maximum speed (70km/h – 43mph). The tram was supposed to decelerate at the 50m marker to 25km/h (15.5mph) but experienced a failure in the braking circuit and the driver didn’t manage to operate the auxiliary brakes in time. The tram derailed at the curve, flipping over and killing five people.
Running in a tunnel with barely any light, then a sharp curve with a stop at the end – the similarities to Sandilands are striking: Three tunnel sections creating first dark, then light conditions. If ATP had been operational, this accident would not have taken place.
ATP monitors the vehicle’s speed at a defined point and, if an excess is detected, will activate the emergency brake independently from human action. Subsequently, the approval processes were accelerated and four weeks later the ATP was running.
Such situations are exactly where technical elements from metro and heavy rail can be of real benefit to light rail. ATP, in this case, helps to relieve the driver from a high level of responsibility under certain operational conditions. SSB took this a step further and not only introduced such systems to its tunnel routes, but also used elements of ATP on some of its street sections and segregated alignments. An example of where this has been especially useful in Stuttgart is at a location with a 7% downhill gradient followed by a sharp 50m curve. It can be tricky for a driver to monitor the speed precisely, so at this point we installed a contact telling the tram to brake down to 30km/h (18.6mph). The braking curve and maximum speed of the tram is monitored in the tram, as it is in a train protection system. If driving properly the operator will not notice the system’s presence, but if not, the system will reduce the vehicle’s maximum speed to the top speed permitted. Our network sports even steeper gradients – up to 8.6%. Such sections are likewise equipped with this device.
From tramway to light rail
Collisions between trams are all too common around the world. In order to reduce the potential for collisions at junctions, Stuttgart’s solution was to combine the signalisation of the points with the proceed/approach signal of the traffic junction. It is a simple and effective strategy, but still not 100% failsafe – in a 2002 collision a driver mistook a signal and ran into an LRV coming from the right. Again, SSB turned to the heavier rail disciplines for a more thorough solution. The signalisations of points and adjacent traffic junctions were synchronised, so that a ‘proceed’ signal will be given only to the direction the point is set.
Further, we like to treat our point signalisation as main signals. A special red designation sign, indicating ‘main signal’ as well the ‘name’ of the signal, means ‘do not pass’ whenever the signal is set at ‘danger’. If a vehicle overruns at ‘danger’, the ATP contact will bring it to an immediate halt. Drivers are now familiar with either situation, following signals or running on line of sight.
Engineering work, blocked lines, and temporary speed restrictions can throw up extra safety considerations. For these, SSB devised warning signals with triangles to indicate any issues ahead, placed at a distance that will allow the driver ample time to react and decelerate accordingly, 50-200m from the hazard. This means the driver always has a clear understanding of what is ahead.
The distance between the warning signal and the actual point where the speed has to be maintained is crucial. For that, we set up some easy-to-check tables.
However, if works are ongoing beyond a sharp corner the tram driver won’t have a chance to see them in time. In that case we announce that the track will be blocked so the driver is ready to stop in time. When the tram arrives at such a situation, someone will then lift the stop signal to allow it to pass safely.
Other features that have helped maintain SSB’s excellent safety record in recent years include infrastructure upgrades to meet modern light rail standards. Since the early 1990s the length of ‘tram’ routes has decreased from 40km (25 miles) to zero (since 2007), while at the same time the total network has increased from 110km (68 miles) to 130km (81 miles). The amount of segregated alignment, tunnels, and grass reservation has also been increased. On-street running is now limited to just 7.8km (4.8 miles). The number of level crossings has also been limited – fewer crossing points means fewer potential interactions with pedestrians and other road users.
These improvements have been borne out in lower incident figures, a decline that directly corresponds with the upgrading of the system – between 1990 and 2007, the total number of accidents almost halved from over 300 to around 180: a clear indication that traffic segregation will improve safety.
A question of responsibility
Although in more recent years the overall accident rate has started to climb again, the figures are slightly deceptive. In relation to increases in distance covered and passengers carried – as well in road use in general (with more car drivers and cyclists) – the figures actually show a decrease.
The backbone, the way of thinking, is to see infrastructure and vehicles as technical hardware; meanwhile, the driver’s skills in operating trams and infrastructure, and the regulations, all work together as a system, leading to maximum reliability. Reliability and safety correspond.
The German word for safety – Sicherheit – encompasses not just the safe interaction of technical elements, but also elements of security, avoiding fraudulent use, protecting the system against attacks, and compliance with the rules. So, Sicherheit suggests a multi-level interaction, reflecting the integrated approach of tramway operation.
To check these measurements, SSB uses the ‘Swiss Cheese Model’, devised in 1990 by James Reason of Manchester University. Several elements or safety barriers form in a consecutive line to prevent an incident from causing harm. The first element of this is time separation. A level crossing with no tram on it is, of course, deemed a ‘safe’ situation. If you add two trams each running to a ten-minute headway, they will occupy the crossing for about one minute, leading to a 1:10 chance of a difficult situation. The remaining 90% of the time, when no trams are approaching the crossing, is completely harmless.
Warning signs, right of way, visibility, vehicle deceleration properties, infrastructure design, the shape of the vehicle, and emergency procedures are all factors that impact on the safety of any potentially dangerous situation. However, we notice some kind of barrier between what we as an industry may influence, and what we cannot.
For instance, the initial event at a pedestrian crossing will put the responsibility on the pedestrian/s to obey the warning signs – will they, or not? If yes, there is no problem. If not, then it is down to the right of way. Will the pedestrians notice that the tram has the right to proceed, and that they have to wait? If not, then for the first time, it is in the driver’s sphere of influence to control the situation. Then deceleration rates, the shape of the vehicle and so on come into play, which puts the responsibility on us as operators. However the initial responsibility for an incident potentially developing into something unsafe is still with others.
Regardless of where responsibility lies, the operator may still be blamed when accidents occur. We are offering a service and so are quite often held responsible for something connected to our service, but which does not lie within our sphere of responsibility.
There is also a real gap between ‘safety delivered’ and ‘safety perceived’: the higher the safety record (which remains very high in tramway and light rail operations), the more people focus on individual incidents. This is something we on the technical side are unable to address – it is something for our PR departments and our marketing teams.
An integrated approach
Technical aspects are just one element of the focus on improved safety, which also require close consideration of the interaction between what is being done at a technical level and the operator running the system.
The regulations governing the SSB are all contained within BOStrab (the German ordinance governing the construction and operation of street railway systems), cf. TAUT October 2010. For an English translation, see www.railforthevalley.com/wp-content/uploads/2010/10/BOStrab-EN_Version_2008-05-12.pdf.
BOStrab describes not just separate components, but also focuses on functional interactions. Several elements are regulated twice – for example, platform height and height of the vehicle floor correspond, and so consequently the regulations of platform height and vehicle floor height are dealt with in two corresponding places.
This approach allows us to harmonise the integration of these elements, shaping the respective elements and their interaction in the best possible way to deliver high reliability and safety.
You have to ensure these elements interact properly, and it is rewarding to follow such an integrated approach. It allows us to steer those points which we, as technicians, may influence, and to address others such as road users and the public in terms of proper behaviour at level crossings, and so on.
Education is also at the heart of SSB’s work and recent initiatives have seen incident figures start to drop again. For example, a recent campaign centred on ‘how to behave at a level crossing’ (www.sicherzufuss.de), and a subsequent survey of around 30 000 pedestrians was undertaken to see how this information had been absorbed. Almost everyone knew about the campaign, and although not all of them behaved properly, that is an issue of personal freedom. Ultimately, that is something that neither SSB nor any other operator has much influence over.
Article appeared originally in TAUT 977 (May 2019).