Why are trains in south London more affected by snow than those elsewhere?

If you think it's train companies in south London that are the worst affected by the weather - you'd be right.

And there is I'm told a technical reason for this.

In south London the companies like Southern Railway, Southeastern and South West Trains have to operate using the "third rail system".

That is, the electricity is transmitted to the train using an electrified third rail that the train picks up through a bit of metal called a shoe.

So, while Network Rail have been running "ghost trains" (empty trains) and de-icing trains through the night there comes a point when it doesn't help.

When the snow and ice hits, if the conductor third rail is cold as soon as snow hits it, it freezes.

That means there's an insulating layer of ice between the shoe and the rail.

Southeastern have told me the trains automatically shut down to prevent "arcing" of electricity - that could damage electrics and is potentially dangerous.

The pertinent point is we are the only country in the world where the third rail system is being used outside an urban area (it's also used on the Tube and has caused problems on the uncovered Metropolitan line in the past).

And why was it introduced?

It was introduced in the 1930s and I've been told it was in the main due to cost as it's cheaper than erecting overhead power lines that we see to the north and east of London.

It also shows why C2C services, for example into Essex which uses overhead power lines, hasn't been as badly affected by similar levels of snow.

According to Wikipedia: "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."

Please let me know if you have any thoughts on this. I would be interested to hear from drivers in particular.

Also, I just received this from Association of Train Operating Companies: