some being in continuous revenue service for more than fifty years. This is especially true compared to internal combustion buses, which tend to require high amounts of maintenance and break down after less than 20 years, mostly due to the vibrations of the engine. In many cases tram networks have a higher capacity than similar buses. This has been cited as a reason for the replacement of one of Europe's busiest bus lines (with three-minute headways in peak times) with a tram by Dresdner Verkehrsbetriebe. Due to the above-mentioned capacity advantage, labor costs (which form the biggest share of operating costs of many public transit systems) per passenger can be significantly lower compared to buses. Trams and light rail systems can be cheaper to install than subways or other forms of heavy rail. In Berlin the commonly cited figure is that one kilometer of subway costs as much as ten kilometers of tramway. ULR Ultra Light Rail developments with prefabricated track and onboard power (no OHL Over Head Line) in the UK are aiming for £10 m per km as opposed to convention tram rail and OHL at £20-£30 m per km Tramways can take advantage of old heavy rail alignments some examples include the Manchester Metrolink of which the Bury Line was part of the East Lancashire Railway. Other examples can be found in Paris, London, Boston, Melbourne and Sydney. They hence sometimes take advantage of high speed track while on train tracks. As tram lines are permanent this allows local authorities to redevelop and revitalise their towns and cities provided suitable planning changes are made. Melbourne will allow higher buildings (5 to 6 story) along tram routes leaving the existing suburbs behind unchanged whilst doubling the cities density. The tram with its fixed route gives developers confidence to invest as opposed to a changeable bus route. Trams produce less air pollution than rubber tyred transport which produce tyre, asphalt and brake based pollutants. The use of regenerative electric motor braking in trams lowers mechanical brake use. Steel wheel and rail particulates are produced but regular wheel alignment and flexible track mounting can reduce emissions. Tram networks can link to other operational heavy rail and rapid transit systems, allowing vehicles to move directly from one to the other without passengers needing to alight. Trams that are compatible with heavy rail systems are called tram-trains, while those that can use