ACT                    The Clapham Junction fire

Some time in the 1950s or 1960s there was a fire at a signal box at Clapham Junction.  It had something to do with a bad fog.  Anyway, Clapham Junction is a major junction in the southwest suburban network of (the then) Southern Railway.  It caused a serious problem.  Whether it was Southern Railway or London Transport that thought of the idea, I don't know,

Southern Railway (SR) use the same voltage as London Transport (LT), but SR have a single-pole and earthed system whereas LT have a double-pole insulated system.  This meant that if a train derailed and shorted one of the two live rails to earth, the system could still carry on.  If this happened with SR, that section of track would have been dead as the breakers at the substation would have tripped out.

So what happened was that LT shorted one of the two rails to earth thus allowing a SR train to run on LT track.  It didn't affect the running of the LT trains but it did allow the SR train to go from Richmond around to its destination via Putney Bridge.  There are all sorts of pieces of interconnection rail that are rarely used, and if you look out of the carriage window you can generally identify these unused rails by the coating of rust on the top of the rail.

I know of another odd rail connection that was rarely used.  During the war an uncle of mine came home from Liverpool on embarkation leave.  He expected to come into one of the north London termini, and then would have to cross London by tube.  But he was pleasantly surprised when the train arrived at Waterloo.  What the arrangement was he never told me, but I would guess that the final destination might have been Portsmouth.