This might answer a bit, I posted this in another thread and copied it to here so you are seeing double!-
Met office is saying the South East will get 40 hours of snow over the next 3 days.
SWT are instigating the snow plan "just in case", if they wait for it to snow the crews and trains will be in the wrong place, there are more trains already running causing congestion at terminal stations as they then need extra drivers (which wont be available at the best of times never mind when its snowing) to get rid of the excess trains and there are then issues if the relieving crews are not available at intermediate stations such as Woking, Basingstoke, Bournemouth etc meaning platforms get blocked and a lot of fed up passengers being left halfway home. All this is why it goes base over apex as it did 2 years ago when they tried to implement the plan at Lunch time which is the last thing they want to be doing.
The snow plan requires-
Less trains and crews so giving a margin for staff who cant get in,
The running times are also slower to allow for the extra driving requirements (extra brake tests, 10mph below linespeed etc) which minimising snow being ingested into the running gear causing failures,
Some services wont run so the points at "delicate" junctions can remain in one direction, its the changing of the points that compacts the snow in the blades causing the failures.
There is more but that will do for now.