This light rail line was promised to be built in the 1930s to connect Kent and Surrey. The route is still covered by London zones and so would help extend tfl in the south.
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Places such as Biggin Hill and Downe have poor connectivity due to the lack of stations there. As it is a tram system it would not be too expensive to build as well as being more environmentally friendly. This is especially true as trams can be used instead of those commuters all travelling by car.