actually be live. Wireless and solid state switching remove the mechanical problem. Alstom developed the system primarily to avoid intrusive power supply cables in the sensitive area of the old city of old Bordeaux. Routes Route patterns vary greatly among the world's tram systems, leading to different network topologies. Most systems start by building up a strongly nucleated radial pattern of routes linking the city centre with residential suburbs and traffic hubs such as railway stations and hospitals, usually following main roads. Some of these, such as those in Hong Kong, Blackpool and Bergen, still essentially comprise a single route. Some suburbs may be served by loop lines connecting two adjacent radial roads. Some modern systems have started by reusing existing radial railway tracks, as in Nottingham and Birmingham, sometimes joining them together by a section of street track through the city centre, as in Manchester. Later developments often include tangential routes linking adjacent suburbs directly, or multiple routes through the town centre to avoid congestion (as in Manchester's Second City Crossing). Other new systems, particularly those in large cities which already have well-developed metro and suburban railway systems, such as London and Paris, have started by building isolated suburban lines feeding into railway or metro stations. In Paris these have then been linked by ring lines. A third, weakly nucleated, route pattern may grow up where a number of nearby small settlements are linked, such as in the coal-mining areas served by BOGESTRA or the Silesian Interurbans. A fourth starting point may be a loop in the city centre, sometimes called a downtown circulator, as in Portland or El Paso. Occasionally a modern tramway system may grow from a preserved heritage line, as in Stockholm. The resulting route patterns are very different. Some have a rational structure, covering their catchment area as efficiently as possible, with new suburbs being planned with tramlines integral to their layout – such is the case in Amsterdam. Bordeaux and Montpellier have built comprehensive networks, based on radial routes with numerous interconnections, within the last two decades. Some systems serve only parts of their cities, with Berlin being the prime example, owing to the fact that trams survived the city's political division only in the Eastern part. Other systems have ended up with a rather random route map, for instance when some previous operating companies have ceased operation (as with the tramways vicinaux/buurtspoorwegen in