Third rails are much cheaper to install and more energy efficient (especially on Southern which were some of the first to use braking to put power back into the system). Third rails are not so often used in rural areas because of the problems with people at level crossings or large animals getting onto the line.

However, overhead power lines do break (usually due to high winds and sometime build up of ice) and the time taken to get trains running again can be much longer. But it is apparently the system used in Scandinavia for most trains because it is more robust in snow and cold weather.