The comfort of passengers, mobility performance, efficiency of transport operations, meeting environmental standards through applied technology, these are all features of low-floor trams. Compared to conventional trams, low-floor trams have determined the increase of passenger flows by reducing station stop times, facilitating the boarding/descent process and encouraging the use of public transport by disabled people. These features have created the premises of new types of demands for passenger mobility which made manufacturers revolutionize technology from the development of bogies to establishing their position so that trams would be performing and capable to support an additional load, as electric equipments are located on top of the vehicle.
Traditional tramways are high-floor trams and access to the tram requires climbing 2 stairs or even 3 stairs to bridge the height difference from the tram stop to the tram floor. Starting around 1980 with a NEOPLAN prototype of a city bus, producers launched activities to avoid the stairs upon entering the vehicles and by 1990, almost all city buses were at least 70% low floor. Quickly both passengers and politicians recognised the advantage of the new concept: it was much more user friendly, it gave much easier access for elderly people and disabled people. After being applied in city buses, it was a natural consequence that the same demand for “user friendliness” affected the development of trams. And this development led to partial low floor trams, it resulted in new technologies for developing bogies and it was then followed by the evolution to 100% low floor buses. From a passenger point of view, of course a 100% low floor tram is offering the maximum comfort: entering the tram floor almost at the same height as the platform height is a decisive plus, in particular for people with limited mobility.
From an operational point of view the 100% low floor tram gives the fastest turn-around time for boarding/de-boarding: the time at tram stops could be shortened because passengers did not have to use 2-3 stairs to board/de-board the tram”, explains Hans-Peter Engel, Sales Director CEE and Russia of Bombardier Transportation (Light Rail Vehicles Division).
Why is urban mobility encouraged by such vehicles? “The low floor trams offer the best accessibility, with floor levels at a height of approx. 350 mm above the track. Better accessibility means also better passengers flow, better exchange rate, educed dual time at the stations and higher commercial speed. Also, with low floor trams, there is no need to build big platforms in the city centres; the platform height is approx. 300 mm providing a better integration of the system in the city centres. In case of emergency, if the passengers have to evacuate the vehicle, it is easier and safer with the low floor vehicle. The accessibility is optimized when the tram is full low floor, with a corridor without any step”, explain Alstom Transport experts.
Technical development upgraded mobility requirements
In the case of standard trams, (6 axles, 3 carbodies), bogies are located centrally under each body shell, while traction motors were located in the car body shells to safe space in the bogies. In combination with relatively large wheels and a very sophisticated hydraulic stabilisation system, such trams showed a reasonable performance, but required a very particular infrastructure. The concept has been revitalised by Solaris and Siemens, the main difference being that Siemens is using “Combino”-bogies with motors in the bogie and Solaris retained the original motor location in the body shells.
In the case of the two types of trams, a major challenge for the development of the modern vehicle was the relocation of the equipment: the classic tram had all the main equipment installed under the floor, but to reduce the floor height it was necessary to locate all main equipment of the roof of the tram – a challenge both for the body shell structure as well as for the dynamic behaviour of the tram. And finally in order to benefit from the low floor a radical change in bogie concept was necessary: the classic stiff axle was dropped, single wheels were applied and the mechanical connection of a wheel pair was replaced by an electronic link. This marked the beginning of modular trams. Instead of having 3 body shells (connected by articulation) resting on 4 pivoting bogies, 5 body shells with articulation/bellows were connected and bogies were installed under body no 1, 3 and 5. This concept gave almost the same ability to cope with curves like the classic tram. The bogies were not pivoting any more, they were semi-fix attached to the body shell.
Such multi-body trams are the most frequently built, however, resignation from classic axles resulted in less good running characteristics and higher wheel wear.
“Bombardier is also using the multi-body-concept – developed and patented bogies which retained the classic axles. To bridge the height of the axles, no stairs are used but slight ramps which do not disturb the passengers so much as stairs. This feature is applied by Bombardier in the “Flexity Outlook” and the new “Flexity 2” trams”, explain Bombardier specialists.
Low-floor trams are very popular due to their flexibility and improved mobility and “ between 80% and 90% of the trams sold today in the world are low floor tramways. But there are some technical difficulties. The available room for the bogies below the floor is extremely limited; that makes the interfaces between the bogies and the carbody complex. Also, all the electrical equipment (transformers, circuit breakers, various cubicles) cannot be mounted under the carbody frame, as it is the case with high floor trams. They have to be integrated on the roof that has to be conceived and reinforced in order to be able to support this additional weight”, Alstom specialists inform.
The new generation of vehicles captures the attention of large cities as they become the symbol of comfortable, efficient and environmental friendly mobility, while high performance, design and flexibility have made producers the decision makers in revolutionizing the passengers’ demands and building a new mobility pattern.
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