First the term power surge is used in the rail industry by folks in the operating department, a true power surge would fry a lot of equipment other than just the signalling. It was more likely to have been a short loss of power, just a few milliseconds is sufficient for the signalling system to go into a fail safe mode.
Why would
NR» say it was an over-voltage fault lasting 20 seconds or more then?
Typically the low voltage supplies derived form the 33kV will change over to a second or even third supply within 2 to 10 seconds, provided there was an actual supply failure, the change over equipment in the substations has a little hysteresis built. Also it can then take the signalling system can take many seconds to start to recover.
The timings in the system are deliberate
