Puddy_Tat
naturally fluffy
In that case you only travel on a short section of the South London Line which was the first main line stretch of railway in the UK to be electrified. It originally had overhead cables (if you look along the trackside you might see the odd chopped-off stump of the gantry structures) but was later converted to 3rd rail system like the rest of the south east.
some nice pictures of the elevated electric on this page (Brighton branch ASLEF)