Trolleybus

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The trolleybus is an electric passenger transport vehicle. Although it resembles a diesel bus, it is not powered by an internal combustion engine, but by an electric motor. Its current is supplied by two overhead cables known as “catenaries,” sometimes called “Overhead Line Equipment” (OLE). This distinguishes trolleybuses from other electric buses that run on batteries. Many trolleybuses are fitted with an auxiliary engine, so they can move at reduced speed in areas without overhead lines, for example when diversions are necessary due to road works, or during parking manoeuvres. Currently, there are about 300 trolleybus networks in operation worldwide. Trolleybuses are sometimes called "trolleys." In the past, they have been called "railless cars," "trackless trolleys" and "trackless trams."

Diagram of a Trolleybus. This model is based on an actual trolleybus still running in Valparaíso, Chile. Model: Pullman Standard type 800, built in 1947, by General Electric Co.

History

First trolleybus prototype of Siemens in Berlin 1882

One year after the introduction of the first electric tram, Werner Siemens produced an electrically operated small wagon. The system was called “Elektromote" and it is considered to be the first trolleybus prototype. The wagon was supplied with current by a small carriage suspended by two overhead lines. The carriage was connected to the vehicle by a flexible cable. This system was called “troller" (from the English word “trawl” itself derived from the Old French word “troller" meaning “to drag"). The name of the system gave rise to the word “trolleybus.” The 540 meter long Elektromote test track in Halensee near Berlin was opened on 29 April 1882 and continued until 20 June 1882. Elektromote pre-dates the first petrol-powered bus, built in 1895 by Carl Benz. There was no further development of trolleybuses in Germany for the next twenty years, although some trials did take place in the United States.

The development of trolleybus services dates from the first decade of the twentieth century, when trolleybuses seemed to be a natural midpoint between trams and petrol-powered buses. At this time, trolleybuses were used not to replace trams, but to connect areas without trams to the tram network. In the interwar period, several cities in North America and the UK converted tram lines to trolleybus use so that the overhead lines could continue to be used even though the rails beneath had worn out. This was because funding for infrastructure was scarce and so that municipally-owned power plants could continue to sell electricity to municipal transport departments. In the 1950s, the rapid development of diesel buses and a reduction in petroleum fuel prices contributed to the scrapping of many trolleybus lines and their replacement with diesel buses.

Advantages

Trolleybuses have an advantage on the steep streets of San Francisco

Trolleybuses are particularly useful in hilly towns where an electric motor is more effective for climbing hills than a diesel engine. The trolleybuses’ rubber tyres also provide better adhesion than do the steel wheels of trams. Trolleybuses are also useful where there is cheap electricity such as occurs in places like Switzerland and Austria where there is plentiful hydro electric power. When standing still, trolleybuses do not consume electricity, and in winter, the electric motor starts easily even at below-zero temperatures. In addition, at high altitudes, the electric motor starts easily on occasions when diesel engines struggle due to rarefied air and therefore less oxygen.[1]

Another advantage of trolleybuses is that they can generate electricity from kinetic energy when slowing down, in a process called regenerative braking. This is a benefit also shared by trams and battery-powered buses. In order to make use of the kinetic energy from regenerative braking, either the trolleybus must be fitted with a battery, or there must be another trolleybus on the same electrical circuit which can use the energy. Trolleybuses, like all electric vehicles, are often seen as more environmentally-friendly than diesel buses. But this is only true where the electricity source is relatively clean. It is not generally the case in Europe where most electricity is derived from burning hydrocarbons.

Disadvantages

Ever since its introduction, the trolleybus has been in direct competition with both the omnibus and the tram. In this context it is sometimes criticised as combining the disadvantages of both.[2][3] The operation of both forms of transport with overhead cables i.e. trolleybus and tram is often criticised as being uneconomical, especially in smaller towns. So, only a relatively easily defined niche market is available to the trolleybus, namely on routes with passenger volumes that are not high enough to justify construction of a tram system, but high enough to render the operation of an omnibus service inefficient.[4] The investment in trolleybus infrastructure is therefore only justifiable on main routes with high frequency service and high passenger levels.[2]

Disadvantages compared with the Omnibus

At bus stations, the provision of individual trolleybus stops is only possible with complex catenary structures

The most frequent criticism of the trolleybus is the higher operational costs compared to diesel or gas-powered buses. Due to the high cost of vehicles and the cost of overhead lines and substations, it is economically inferior to the omnibus.[5]

The trolleybus shares with rail systems the restrictions of a physically defined route. Re-routings, temporary or permanent, are not usually readily available outside of "downtown" areas where the trolleybuses may be re-routed via adjacent business area streets where other trolleybus routes operate. This problem was highlighted in Vancouver in July 2008,[6] when an explosion closed several roads in the city's downtown core. Because of the closure, trolleys were forced to detour several kilometers off their route in order to stay on the wires, leaving major portions of their routes unserved and service well off schedule. Without additional overhead infrastructure it is also impossible to run detours or shortened routes during less busy periods, as is usually the case in many cities with omnibuses during the evenings, at night and on weekends. In Solingen six regular trolleybus lines are replaced overnight by six night-express omnibus lines, all serving routes which deviate from their daytime service. This sometimes leads to confusion among passengers[7]

Furthermore the operation of school services for both the start and the finish of the school day is not feasible if these deviate from the regular route. The same applies to servicing industrial sites at the times of shift changes.[8] It is particularly uneconomical to reserve expensive trolleybuses for school transport. They are used only once or twice a day and have the additional disadvantage of being unsuitable for any class outings which are outside the cabled route. Consequently they cannot be used away from the regular route on school trips, club outings or any other such, and are thus precluded from bringing the operator any revenue from non-scheduled services.

Another cost factor is the provision of diesel buses as an operating reserve in order to run an emergency service in the event of disruption to the infrastructure.[9] This is analogous to a rail replacement bus service.[10][11] Typical examples of disruption are construction sites, construction and maintenance work on the overhead cables, traffic accidents, power outages, lightning strikes, strong icing of the catenary, storm damage to the catenary, careless excavator operation, and overloaded lorries. In La Chaux-de-Fonds, for example, there are 15 such disruptions annually which necessitate a replacement bus service.[12] If there is not sufficient operating reserve, omnibuses need to be hired from other operators[13][14] which is often not possible at short notice and may result in longer operating restrictions on the trolleybus route.[15]

A trolleybus overtakes another trolleybus becoming dewired as a result

Trolleybuses cannot overtake one another in regular service unless two separate sets of wires with a switch are provided or the vehicles are equipped with off-wire capability. The video on the right shows a trolleybus becoming dewired when it overtakes another trolleybus. The inability to overtake in regular service prevents the provision of express services.[16] Furthermore, empty vehicles returning to or rejoining service from the depot cannot take the quickest route. This means they cannot for example, use bypass roads but must follow the regular route. To avoid this, in some towns extra route bypass infrastructure is provided. These in turn are comparatively expensive to maintain because of their infrequent use and lack of revenue generation from fares. Passenger direction-dependent services - for example, in the morning on the regular route into the city and then empty as fast as possible back to the start and vice versa in the afternoon, are also difficult to achieve with trolleybuses without complex additional infrastructure. An example of this is the Zurich line 46, where the frequency is increased by means of diesel buses.[17] Trolleybuses are also unable to turn at any point in the network in the instance of technical difficulties. By contrast, an omnibus can turn at any major intersection. Furthermore, trolleybus services cannot be supplemented by the use of any extra units to cover demand at major events, because the capacity of the substations is usually designed for regular operation only. In St Gallen, a new substation had to be built when the local transport operators introduced double articulated trolleybuses on the regional network.[18]

Transport operators who use both trolleybuses and omnibuses have complex issues of personnel disposition as there need to be separate rotas for drivers with, and drivers without a trolleybus licence. In certain circumstances this can lead to omnibuses having to be used simply because there are no drivers available with a trolleybus licence.[19][20] Should a transport operator desire fully flexible use of employees, then the entire pool of drivers must be trained to drive trolleybuses despite the additional cost. This is the case for example with the Esslingen Municipal Transport Company[21] even though they operate about three times as many omnibuses as trolleybuses.

A trolleybus stranded by a dewirement blocks a junction. The resulting damage to the cables disables the following trolleybuses

For technical reasons not every bus line can be electrified: Level crossings with electrified railways or particularly low underpasses both exclude trolleybus operation. An example here is the underpass at the Wuppertal-Vohwinkel station. This prevented the extension of the Solingen line 683 since the necessary lowering of the roadway would have cost around four million euros.[22] The same applies if sections of the route run along motorways or other fast roads with a minimum speed limit of 60 or even 80 km/h. Also trolley buses have unique operating characteristics, such as trolley drivers having to slow down at turns and through switches in the overhead wire system.[23] This is to prevent the trolleybus's pantograph becoming disconnected from the overhead cable. In Geneva there is a turning loop with a speed limit of only 10 Km/h.[24] At these places they obstruct the free flow of traffic[25] and also extend their travel times relative to diesel buses. Dewirements—when the trolley poles come off of the wires—sometimes occur, especially in areas subject to heavy snow. After a dewirement, trolleybuses not equipped with an auxiliary power unit (APU) are stranded without power. When approaching switches, trolleybuses usually must decelerate in order to avoid dewiring, and this deceleration can potentially add slightly to traffic congestion. In the case of a dewirement, an entire junction may become blocked. Some trolleybuses are fitted with an auxiliary engine, so they can move in areas without overhead contact lines, for example when diversions are necessary due to road works, or during parking manoeuvres. A consequence of this is reduced speed. Trolleybus networks are divided into sections each of which is supplied with electricity by a nearby electrical substation.

A trolleybus route shown in blue, connects with diesel bus routes on the edge of town.

A trolleybus system does not allow multiple branch routes from rural suburbs to be integrated into high frequency central area corridors, as the infrastructure required is unjustifiably expensive. If it is desirable to run an exclusively trolleybus system in the central area, then passengers from these suburbs will be obliged to change vehicles.[26] These broken journeys involving other transport modes make the trolleybus an unattractive option.[27] This problem is illustrated by the diagram on the right, the notional trolleybus corridor is shown in blue. The problem is similar for single routes that run less frequently at the periphery than in the centre.

Trolleybuses only have a valid claim to being environmentally friendly when the electrical power used is generated from renewable energy sources. If on the other hand it derives from coal-fired power plants, steam power plants, oil-fired power plants, gas turbine power plants or waste incineration plants, then the emissions are merely shifted to another location. If the power source is a conventional coal power plant, the carbon footprint of the trolleybus is actually larger than that of the diesel bus.[9] A joint study by Winterthur Municipal Transport (CH) and the Swiss Federal Office of Energy confirms this. Accordingly, the carbon footprint of a trolleybus using electricity from a typical European primary energy mix is not significantly better than that of a diesel bus, thereby undermining any justification of the 24% higher operating costs [28] If the primary energy source is nuclear then this reduces its acceptance in many sections of society.[9] The problem of nuclear waste disposal exists elsewhere than at the point of use. In the immediate vicinity there is the problem of electromagnetic pollution.[29] The decisive factor in the carbon footprint of the trolleybus is therefore the primary energy mix used in the generation of the electricity used. A negative example here is the trolleybus in Tallinn. As late as 2004 the primary energy source in Estonia was still shale oil.[30] The chemicals used to de-ice the overhead cables are also an environmental disadvantage.[31]

On the other hand, diesel buses have also become more environmentally friendly over recent decades, partly as a result of stricter emission standards - such as the Euro-standard in the European Union - and also through improved noise encapsulation of the engine. This further diminishes any environmental advantage the trolleybus may have had. Additionally, as a result of further future tightening of standards, we can expect a marked reduction of polluting emissions from diesel bus engines.[2] In global terms, the contribution of the trolleybus to climate protection is negligible,[32] when one considers that compared to a worldwide total of approximately 600 million motor vehicles there are a mere 40,000 trolleybuses.

The overhead cables are sometimes seen as being unaesthetic, here in Luzern

Some trolleybus systems have been criticised for aesthetic reasons, with city residents complaining that the jumble of overhead lines/overhead wires was unsightly.[33] This is often the case in historic town centres. This is especially true for complex cable arrangements at branches and intersections. Intersections often have a "webbed ceiling" appearance, due to multiple crossing and converging sets of trolley wires. The same goes for the often massive catenary masts, especially if they have to be placed in the middle of pavements.[34] This is often referred to as visual pollution.[35] Fire services sometimes complain that the overhead cables prevent the use of turntable ladders in narrow streets[36] Similarly the use of cranes is made difficult. Additionally the overhead cables must always be adapted to the new traffic conditions during road alterations often with high associated costs, even if this only entails re-labelling of traffic lanes. The demolition of buildings adjoining the route may necessitate the catenary rosettes being replaced at the expense of transport companies by temporary poles.[37] The construction of new lines brings a high risk of litigation by individual residents who object to the installation of the necessary infrastructure. This is especially true in respect of catenary installation in rural areas. [38]

Due to the long service-life of trolleybuses, innovations in vehicle construction cannot be implemented as frequently as with diesel buses. This is illustrated by the fact that many cities still operate high-floor trolleybuses, where the omnibus fleet has long since been converted to low-floor vehicles. Another example is the development of passenger information displays, which has left many trolleybuses operating roller-blind displays which are now considered outdated. The long life of a vehicle fleet can be detrimental to passenger satisfaction.[2] In addition, the maintenance and operating costs of an older vehicle are higher than that of a newer one.[39] In order to meet the changing needs of passengers and transport operators, many trolleybuses are frequently retrofitted for their last years in service.[40] As vehicles age, the procurement of spare parts becomes more and more problematic especially when electronic components become obsolete after a few years. Modern trolleybuses no longer have the life expectancy of the technically simpler but robust classic old timers hence a further loss of advantage over the omnibus. In contrast, omnibus and coach engines have become more reliable over the decades, and accelerate faster than previously.

Trolleybus power lines in Sarajevo destroyed in the Bosnian war

Furthermore, the higher unladen weight of a trolleybus affects the maximum permitted number of standing passengers, which is determined by the combined mass of vehicle and passengers. In Germany for example, a three-axle articulated vehicle must not weigh more than 28 tons. So the 1963 vintage 600kg articulated trolleybuses of the Offenbach municipality were only allowed 104 standing places compared to 114 standing places for the structurally identical diesel bus equivalent.[41]

Due to the high voltages and currents necessary for powering the trolleybus, there is a risk of fire in the electrical system. A certain amount of this risk is due to voltage surges caused by lightning strikes to the overhead cable. Consequently, thunderstorms often bring with them significant operational disruption. Damage here can be to both stationary elements such as cable junctions and to any vehicles caught in the area of excessive electrical charge.[42] With rising commodity prices for non-ferrous metals, especially in economically weaker states, the risk of catenary theft during breaks in the service such as those that occur at nighttime is increasing. This can lead to prolonged gaps in the serviceable route and causes additional repair costs.[43][44] Furthermore, trolleybus lines are more susceptible to the effects of war and terrorism than omnibus lines and correspondingly, in contrast to omnibuses, have no potential civil defence utility.[45]

Problems arise in winter, when high electrical resistance develops due to icing. Frost forms on the overhead lines when the temperature falls to below two degrees Celsius and the humidity rises to over 70 percent. This can lead to the failure of individual sections or a complete breakdown of the network. Trolleybuses with traditional controls and without voltage monitoring can sometimes continue to operate in freezing temperatures but at lower velocities, although wear is greatly increased due to burning of the contact shoe. Icing causes particular difficulties for more modern trolleybuses with sensitive electronic controls. These are susceptible to power interruptions and the resulting arcing. In addition, the arcing causes the copper contained in the overhead lines to evaporate, thus accelerating its wear. Furthermore, the overhead lines can break under the weight of heavy ice. In many cities, in order to free the overhead lines of ice, vans and even buses equipped with contact poles scrape the ice from the cables, whilst spraying them with anti-freeze.[46] These operations are carried out at night or early morning, shortly before the first trolleybus leaves the depot. As an alternative to this technique, trolleybuses equipped with poles that spray anti-freeze can regularly spray the overhead lines during the night to prevent freezing.

The use of snow chains is usually not possible with trolleybuses, as this can result in a short circuit between the overhead line and the snow on the road or other electrically conductive elements on the road surface. Instead, some trolleybuses are fitted with a gritter in front of the drive axle to increase traction on slippery surfaces. These fixtures can also be used on snow-free roads, for example, roads covered with fallen leaves, but they are less effective than snow chains. Therefore, many transport companies increasingly use twin-engined articulated trolleybuses that have better handling on snow.[47] Nevertheless, single-engined trolleybuses are still used on important routes but with steep sections temporarily shut down. Alternatively, twin-engined articulated trolleybuses or solo trolleybuses with or without snow chains are used.[48][49][50]

Statistics gathered by the National Transit Database of the Federal Transit Administration show that in the United States between 2008 and 2012, a trolleybus was twice as likely as an omnibus to injure a cyclist, and three and a half times as likely as an omnibus to injure a pedestrian.[51] When trolleybuses used to run in the United Kingdom, they were nicknamed "Silent Death,"[52][53][54] "Silent Killers"[55] and "Granny Killers."[56] In Australia they were known as "Whispering Death."[57]

Disadvantages compared to the tram

Without bus lanes, trolleybuses and omnibuses are completely dependent on the flow of traffic

A common disadvantage of both the omnibus and trolleybus is the dependency on road traffic conditions. Where there is no bus lane available there is the constant possibility of being stuck in a traffic jam.[58] On the other hand, where bus lanes are created these require more space due to greater vehicular width than does a segregated tram line. Furthermore, the bi-polar cable system for trolleybuses is both more complex and more susceptible to technical problems (than the single cable on tramways)[59]

When experiencing technical difficulties the trolleybus is also less flexible than a tram which can in exceptional circumstances be steered from the rear back to the nearest set of points or junction. In addition, trolleybuses need space intensive turning loops at the end of the routes which are not needed by reversible tram units. And because they run on rails, trams are able to take tight corners. Unlike trolleybuses, trams can share railway track with trains. Tram pantographs don’t detach from overhead cables, whereas trolleybus pantographs often do. Trams require just one overhead cable as the return of power takes place through the metal rails. Because trolleybuses are fitted with rubber tires, the return of power takes place through a second catenary parallel to the first. The extra overhead cabling means that trolleybuses cause more visual pollution than trams.

A bi-articulated trolleybus in Zürich. Vehicles of this length are prohibited on German and UK roads.

Like the omnibus, the trolleybus has a poor passenger capacity in comparison with trams or trains. Even an articulated vehicle can only carry about 150 persons. By contrast a 75m long tram or light rail train can transport up to 500 passengers at a time. Multiple unit trolleybuses are only feasible in limited circumstances, in Germany and the UK for example, they are not permitted. This raises the staff to passenger ratio in comparison to trams.[2] Trams can adapt to demand by adding extra carriages during the rush hour (and removing them during off-peak hours). This is not possible with trolleybuses. When it comes to passenger comfort the trolleybus is also at a disadvantage when compared to a tram. Although the horizontal movement is similar to a train, it suffers the same vertical oscillations as a diesel bus.[2] And the smoothness of ride on a trolleybus is dependent on the smoothness of the road surface. The adhesion conferred by the tires gives the trolleybus a potential acceleration/deceleration better than a tram, enabling it to climb over steep slopes. Conversely, the energy consumed is proportionately larger for the displacement. A tram consumes only 53 percent of the energy consumed by a trolleybus per passenger.[60] Furthermore, if the road surface on which the trolleybus runs is not strengthened, it will regularly have to be re-laid because of the phenomenon of "rutting.” Rutting is caused by the necessity for trolleybuses to follow the same track in order to prevent their pantographs becoming detached from the overhead cables. Rutting is worst at bus stops with level boarding.

Prospects

The decline of trolleybus numbers in the United Statesl

The future for trolleybuses is not bright. They continue to operate, especially in the former Soviet bloc , where they were established in almost all cities with more than 200,000 inhabitants. But even here, the numbers have fallen dramatically from 31,148 trolleybuses in 2000, to 21,928 in 2012 - a drop of 30% in 12 years. They still have a role to play in hilly areas and where electricity is cheap and clean, but the global trends are negative. There were 40,665 trolleybuses operating worldwide in the year 2000. By 2012, this figure had reduced to 27,814 - a drop of 32% in 12 years. In the United States alone, the number of trolleybuses has declined from a peak of over 7,000 in 1952, to just 570 in 2012. Nobody likes the overhead cables, and the problem of urban air quality is being tackled by other means. In an attempt to reduce the visual pollution caused by overhead lines, Geneva is experimenting with flash-charging battery-powered buses. These are charged in just 15 seconds at every stop and in 3 to 4 minutes at the terminus. With battery-powered buses becoming more advanced, the trolleybus may become a heritage asset rather like cable cars.This has already happened in Chile, where the trolleybuses in Valparaiso (the only network in the country) have been declared part of the country’s cultural heritage.[61]

Preservation

Across the world, numerous transport museums, associations and individuals seek to preserve historically valuable trolleybuses as well as the related infrastructure. Old trolleybuses are a monument to the past, and objects of interest for today. The oldest surviving trolleybus was built in 1922 and is a non-operational vehicle from Toronto in Canada. The oldest surviving European trolleybus is also non-operational. It's from Keighley in the UK and was built in 1924. The oldest functioning trolleybus is a 1931-built vehicle from Christchurch in New Zealand. The oldest functioning trolleybus in Europe is from Lausanne and was built in 1932. In many cities, vehicles held in museums leave the museum on short excursions. In the UK, New Zealand and the United States there are even independent museum installations with specially constructed catenary.

References

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Acknowledgements

Bibliography

  • Grava, Sigurd 2003. “Trolleybuses.” Pp 421-436 in Urban Transportation Systems, (c) The McGraw-Hill Companies, Inc. ISBN 10: 0071384170 ISBN 13: 9780071384179

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