I remember the Garswood terminators in the days of MPTE &
City Line branding - probably late 1970s and into 1980s.
There was never a turnback siding at Garswood - the reversal was done via a signalled trailing crossover on the Bryn side of Garswood station.
This diagram (from the commissioning of Warrington PSB in 1972) shows the signalling arrangements.
View attachment 204212
At the time, traffic on the line via St Helens was quite light - an hourly Lime St / Wigan NW all-shacks stopper plus, on the half-hour, the all-stops Lime St / Garswood. There were no limited-stop or longer-distance trains going beyond Wigan and freight was pretty sparse, so no real issues reversing a DMU on the main line.
The crossover at Garswood must have been removed some time ago. I had guessed during the electrification project - but it had been eliminated by the time of my 1990-issue Quail map.