I'm not sure about the rest of the network, we were too busy trying to
get things moving our end, but the problems at the northern end of the
Met were caused by a stalled train at Chorleywood northbound.

Evidently it was a combination of the snow on the track being crushed
into ice by the train, and isolating the electric shoes, and the wheels
spinning which in turn burned out the motors!

To (eventually - about 8am was our first) get the met running again
another train had to push the stalled one al the way to Amerham, so we
enjoyed the fun of a 16 car train trying to reverse from platform 1 -
not the most standard manouver. Until this was out of the way there
hadn't been a single northbound train, which of course compounded the
delays.