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Tram design: issues and constraints

Lisbon’s tramway is effectively two systems: extreme slopes, curves and clearances where only the old-configuration four-wheelers can go, and the low-level riverside line.

The electric tramcar has been with us for about 125 years now, and in most areas of tramway operation development has proceeded with a number of significant step-changes in fundamental design over those years.

Starting with the simple four-wheel car, often coupled in trains with trailers, the next generation saw development of the bogie car, followed by the articulated car and culminating (so far) with the ‘low-floor’ car. The US followed the first part of the pattern, going from Frank Sprague’s ‘infant sparkers’ to the sophistication of the PCC car in around 50 years before economic and political developments killed the US street railway industry.

In the UK the trailer was not favoured, and after some initial rather half-hearted experiments, additional passenger accommodation was loaded on the roof to produce the typical British double-deck car. Some remarkably sophisticated cars were produced in the 1930s, but again economic and political developments effectively killed the tramway industry around the time of the Second World War.

The German Concept

It is only in the nations of Mitteleuropa, particularly Germany, that all stages of tram development can be followed. The four-wheel motorcar and trailer had a remarkably long career here, stretching from the dawn of the industry to the KSW and Verbandstyp cars of the 1940s and early 1950s. The development of the Grossraumwagen allowed Germany to move on to the more economic and comfortable bogie car, and this was followed in 1956 by the production by the Düsseldorfer Waggonfabrik (Düwag) of large numbers of articulated tramcars for operators in Germany and abroad to replace old pre-war models.

‘Low floor’ trams were developed to provide easier access for passengers, particularly those with disabilities or encumbered with baby carriages or luggage. Initial schemes had cars with low-floor sections, but in the 1990s the German firm MAN produced what was claimed to be the first ‘100% low-floor tram’. Since that date all car builders have sought to produce low-floor cars.

The ‘German Concept’ has been followed in many countries, to the extent that the articulated tram has become the default option for most tramways and is adopted unthinkingly by the promoters and advisors for most new tramway projects. But is it the best option? Some examination of the economic situation in Germany in the 1950s and 1960s can explain why it seemed appropriate there and at that time, but can raise questions as to its validity in other places and at other times.

German regulations required that coupled trains of trams should have a Schaffner (conductor) on each individual car; thus a three-car set would have a crew of one driver and three conductors. In a period of rapid economic growth and manpower shortages it became difficult and uneconomic to recruit staff. Replacing a three-car train with a single articulated car reduced staff to two – driver and conductor; later only the operator who combined driving with limited conducting.

Challenges and issues

The downsides of large articulated cars were overlooked in this development; they are more complex to build, can make infrastructure more complex and for much of the day are running only part-full. This last issue can make the tram less environmentally-friendly and lead to risks of onboard anti-social behaviour.

It is arguable that these problems could be avoided by the use of coupled sets of simpler bogie cars; modern technology can provide a data-bus linked through all cars in the train, allowing the operator to monitor CCTV in all parts of the train and to operate two-way communication with passengers.

System operators will tell stories of the complexities of making and breaking trains throughout the day, but modern automatic couplers and sensible scheduling to ensure that trains pass depots or parking sidings should ensure that this can be managed.

Low floors = Level boarding

If the adoption of large articulated cars caused problems then the adoption of low floors has arguably created even bigger ones. There can be no doubt that level boarding is a huge benefit to all users of public transport; accidents whilst boarding and alighting vehicles with steps have always been a significant element in public transport risks – eliminate the steps and the risk diminishes significantly. Level boarding can reduce stop dwell time, thus improving commercial speeds without raising vehicle speed limits, and level boarding benefits a significant proportion of passengers. However these benefits bring with them a number of problems that need to be considered.

A high-floor tramcar has significant space below the floor for the location of equipment boxes; reducing the space and equipment that has to be relocated either in cabinets within the car or on the roof. Cabinets within the car will reduce space for fare-paying passengers, although there can be significant advantages in weatherproofing and maintenance access if complex machinery is located within the clean environment offered by the car’s weather enclosure. If the equipment enclosures are well thought out, as is the case of the AnsaldoBreda cars used in Lille, then the benefits can mitigate the theoretical disadvantages.

Locating equipment on the roof brings many problems; the car’s structure has to be made stronger to support the weight, adding to the overall mass. The raised centre of gravity will also increase the car’s tendency to roll – if the car is to be kept within a tight kinematic envelope this will mean that the suspension has to be stiffened, resulting in a harsher ride for the passengers.

Going round the bend

The sharpness of tramway curves has a major effect on design, by limiting the maximum wheelbase achievable for both four-wheeled and bogie tramcars. The longer the wheelbase, the more likely a tram is to derail when rounding a sharp curve. However, the shorter the wheelbase, the more likely that the tram is to ‘hunt’ or yaw from side to side – sometimes this oscillation mode can be quite violent on four-wheeled tramcars.

It is notable that the history of tramcar development is littered with attempts to overcome this problem with the radial axle truck and individual, self-steering wheels being tried.

Attempting to achieve a low floor usually means that conventional bogies cannot be used. The conventional bogie, particularly one with a floating bolster, can significantly improve ride characteristics. The bogie has the effect of halving the jolt transmitted by railhead misalignments and the bolster mitigates horizontal jerks – allowing the car to feel as though it is floating. The bogies can also swivel to a large extent, allowing the car to negotiate tight bends – indeed some of the older systems have bends as tight as 11m. Whilst such bends bring noise and wear problems and should be limited in the planning of new tramways, the modern convention that bends should be no tighter than 25m can bring huge problems when attempting to insert commercially attractive tramway routeings into established cities.

Lowering the bodies around the wheelsets means that a conventional bogie is often impossible to achieve. In some respects the advent of modern low-floor trams is a step back to four-wheeled tramcar designs, due to the design constraining the isolation of the body from the movement of the wheelsets. The leading body unit of both of these tramcar designs has a marked tendency to hunt at higher speeds, and the ride quality is quite harsh in comparison to older bogie tramcar designs. Attempts to dampen the ride have been made by manufacturers, but the inevitable problem is that stresses are consequently passed to the track – leading to increased wear – or are passed to the body sections, leading to structural failures.

Inevitably, low floor trams with fixed bogies experience increased track wear and tear, while decreasing the speed at which a tram can drive through a curve (usually 4-15km/h in 20m radius curve). Some public transport authorities report that low-floor trams have 15% higher maintenance costs for the rolling stock, and 20% higher maintenance costs for the infrastructure on average.

It would appear that the current attempts to achieve a 100% low-floor car lead to numerous problems, resulting in a service offering to the public that is less comfortable to ride and an operating regime that is less economically attractive than the previous generation of cars. Finding solutions to these problems will be the challenge of the next decade.

For the full version of this feature, please see Tramways & Urban Transit – January 2015 issue (925).