Conversions
Despite various technical possibilities of operating stock with dual power collecting modes, the desire to achieve full compatibility of entire networks seems to have been the decisive cause of conversions from third rail to overhead supply (or vice versa).
Suburban corridors in Paris from Gare Saint-Lazare, Gare des Invalides (both CF Ouest) and Gare d'Orsay (CF PO), were electrified from 1924, 1901, 1900 respectively. They all changed to overhead wires by stages after they became part of a wide scale electrification project of the SNCF network in the 1960s–70s.
In Manchester area, the L&YR Bury line was first electrified with overhead wires (1913), then changed to third rail (1917, cf. Railway electrification in Great Britain) and again in 1992 to overhead wires in the course of its adaptation for the Manchester Metrolink. Trams in city centre streets, carrying collector shoes projecting from their bogies, were considered too dangerous for pedestrians and motor traffic to attempt dual-mode technology (in Amsterdam and Rotterdam Sneltram vehicles go out to surface in suburbs, not in busy central areas). The same thing happened to the West Croydon – Wimbledon Line in Greater London (originally electrified by the Southern Railway) when Tramlink was opened in 2000.
Three lines of five making up the core of Barcelona Metro network changed to overhead power supply from third rail. This operation was also done by stages and completed in 2003.
The opposite took place in south London. The South London Line of the LBSCR network between Victoria and London Bridge was electrified with catenary in 1909. The system was later extended to Crystal Palace, Coulsdon North and Sutton. In the course of main-line third rail electrification in south-east England, the lines were converted by 1929.
The first overhead electric trains appeared on the Hamburg-Altonaer Stadt- und Vorortbahn in 1907. Thirty years later, the main-line railway operator, Deutsche Reichsbahn, influenced by the success of the third-rail Berlin S-Bahn, decided to switch what was now called Hamburg S-Bahn to third rail. The process began in 1940 and was not finished until 1955.
In 1976–1981, the third-rail Vienna U-Bahn U4 Line substituted the Donaukanallinie and Wientallinie of the Stadtbahn, built c1900 and first electrified with overhead wires in 1924. This was part of a big project of consolidated U-Bahn network construction. The other electric Stadtbahn line, whose conversion into heavy rail stock was rejected, still operates under wires with light rail cars (as U6), though it has been thoroughly modernised and significantly extended. As the platforms on the Gürtellinie were not suitable for raising without much intervention into historic Otto Wagner's station architecture, the line would anyway remain incompatible with the rest of the U-Bahn network. Therefore an attempt of conversion to third rail would have been pointless. In Vienna, paradoxically, the wires were retained for aesthetic (and economic) reasons.
The western portion of the Skokie Swift of the Chicago 'L' changed from catenary wire to third rail in 2004, making it fully compatible with the rest of the system.
The reasons for building the overhead powered Tyne & Wear Metro network roughly on lines of the long-gone third-rail Tyneside Electrics system in Newcastle area are likely to have roots in economy and psychology rather than in the pursuit of compatibility. At the time of the Metro opening (1980), the third rail system had already been removed from the existing lines, there were no third-rail light rail vehicles on the market and the latter technology was confined to much more costly heavy rail stock. Also the far-going change of image was desired: the memories of the last stage of operation of the Tyneside Electrics were far from being favourable. This was the construction of the system from scratch after 11 years of ineffective diesel service.
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