The average tram vehicle itself does not differ much in specifications from many of the LRV's used in U.S. cities today. There is however a difference in implementation; in short, in order to win local referendums to move forward on a rail system, proponents in the U.S. have turned light-rail into heavy-rail, while continuing to call it 'light' for public relation purposes. Thus, while American systems purchase 'light' rail vehicles, they use them primarily in a heavy rail infrastructure.
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| Siemens ULF in Vienna |
While Vienna has one of the largest tram systems in the world, it is but only one of two systems which use this model, manufactured by Siemens. This highly limited number of users raise issues which have to be considered as well.
The model's name is simply ULF, an acronym for ultra-low floor. It is the lowest floor available among rail vehicles today, and is also 100% low floor. To produce it, it was necessary to revolutionize the method in which the wheels interact with the drive motors. Virtually all rail vehicles of any type have, for over a century, used bogies with double axles and wheels affixed at each end, with typically two bogies for each rail carriage.
The ULF vehicles use single wheel sets, and forego the double wheeled axles, instead having each wheel driven by its own motor. The motor assemblies are positioned above the wheels. By eliminating the central axle and motors space is freed up to position a lower floor for the carriage assembly. An additional advantage is that the single wheel sets make possible a much tighter turning radius than can be had with the traditional use of bogies.
There are several low floor trams models, either partial or 100%, in use in many cities, including in the U.S. The most common floor height above rail (street surface) for this class of trams is 350mm (13.78"), while a few achieve a floor height of 300mm. The ULF floor height is 220mm (8.66"), with the entry height, slightly lower, at 210mm (8.27").
The 8.27" entry height means that a typical curbside and sidewalk would suffice to provide ease of entry/exit for passengers of any mobility status. This has a great impact upon expenditures on infrastructure needs. Few stop locations will need enhancements to accommodate the tram operation.
The low floor also increases the speed of entry/exit for even the most able of passengers, reducing stop times significantly. Reduced stop times benefit the system by shortening trip times, thus attracting more ridership, and by reducing the total number of trams needed to provide a given level of service. Since the vehicles are the most costly ongoing capital expense item of a transit system, this factor translates to nice savings.
The turning radius benefit is of significance, providing greater flexibility in routing the system and in-turn fewer infrastructure adjustments to make, and thus more savings. The single wheels sets are also likely to provide significant reduction in noise when a tram makes a turn. Bogies with two wheel sets, unless meticulously engineered, create intense metal on metal friction causing shrieking noise at turn locations. This same action greatly wears down the rails themselves, a costly effect over a long period of time, and one that produces more frequent disturbances to repair.