Andy,
Not too sure what you're referring to here. The only area in Kent that I know that involves switching between a third rail supply and the overhead is at the entrance to the Channel Tunnel where a France/Belgium bound train raises its pantograph and lifts its shoes prior to *entering* the tunnel.
There are maximum and minimum heights for the overhead wire above the train and sometimes through tunnels the wire is pretty near its minimum. A classic example is the Manchester Metrolink tram (trolley) system whose panographs reach up for the wire in town (a maximum for safety) but then squeeze through Heaton Park Tunnel (ex-railway) with the wire and tram in closest proximity.
Where space is at a premium, a double track railway can be singled and slewed to the centre of the tunnel to take maximum advantage of the arch.
[This message has been edited by Hutch (edited 11-12-2002).]
