When was the avoiding line done away with?
The Wigan Avoiding (Whelley) line closed to regular traffic when mechanical signalling was removed & the northern part of Warrington PSB area was commissioned on 1st October 1972.
However the Whelley line and the slow line from Standish Junction to Balshaw Lane Junction was left in place for diversions until completion of the electrification project between Weaver Jn and Preston in July 1973.
After the start of electric trains, the Whelley line lingered on as a basically disused single line between Balshaw Lane and Bamfurlong for a few years until formally closed & lifted in the mid-70s.
This is the reason for today's headspan supports on the 2-track section between Standish and Balshaw Lane Junctions - the electrification overhead had to be installed (but was not energized) during 1971/72, with the slow lines still in place and in regular use.
It has only ever been double track between Wigan NW and Standish Jn (although there were one or two locations with sidings, loops etc which served coal mines to the west of the WCML).
One other piece of trivia - today's Balshaw Lane Junction, where the slow and fast lines converge, was only established at the time of re-signalling/electrification. In steam days, the slow and fast lines ran as parallel but unconnected pairs of tracks from Standish Jn, through Coppull to Euxton Jn. Pity BR did not just rebuild Standish Junction in 1972 and have 4 tracks from there through to Preston.
At the time all this happened, I was an eager, bike-riding teenage trainspotter, living in the area. I still remember the horror turning up trackside with my mate one Saturday afternoon in late 1972, and seeing gangs of workmen chopping down all the semaphore signals, and disappointment when there were no more loose-coupled goods trains hauled by Class 25s or Class 40s clanking along the Whelley line.