Barry and All, You may care to click on this You Tube link (6 minutes and turn the sound up):
It shows a Southeastern train in real difficulties down in Strood, attemting to get around the loop to Rochester during significant snowfall in early December, 2010. The driver must have had the patience of Job but you will certainly see the local difficulties of the third rail in such conditions that perhaps even Network Rail cannot easily cope with on the spot. It is not something that the railways are proud of but it is certainly something that has beset the third rail system since its inception. I certainly have sympathy in the face of such adversity. I know it is also the case that, more recently, one by one, the points on the Herne Hill/Tulse Hill/Streatham routes gave out one by one as they were moved at the commencement of the early morning service because snow was compacted in the pointwork by movement of the point blades when they were first moved to switch directions. That effectively stalled the Peckham Rye - Tulse Hill services as well. When the previous overnight 'ghost trains' were run, they were run without requiring the points to be changed. A lesson to be learned there, I feel.