The reason why GWR▸ could update over a 100 miles of track in one weekend in 1892 was because for several years they had been updating the track to allow dual - gauge.
It was a relatively simple job to remove the third rail to leave a standard gauge track.
Whilst not trying to diminish GWR's achievement, the job was not a weekend wonder, but planned and implemented over a number of years.
Plus of course they could mobilise cheap labourers in large numbers of to undertake the actual changeover.
The interesting point is when did they do the preparation and did they have blockades and a large number of possessions to do the work? My feeling is that most of the preparation would have been done by the local
PW▸ gangs between trains.
Did you know the
LNER» poineered a single line track relaying machine before the war? With all the mechanisation of PW work we don't seem to have progressed too much as we now have to close both lines to relay track.