Staff to a6et.
It is between Helensburgh and Waterfall but I do not know whether it is Mug's Flat as I do not know where that is.
My recollection is that it was actually near the top of the grade near the deep cutting where the original tunnel was opened out but it was 40 odd years ago.
I suspect that Mug's Flat is the level straight section where the line from Helensburgh winds out of the valley and crosses the end of the ridge before curving back into the valley to the "U" turn near the north end of Cawley tunnel. If that is the case I can understand it being known as Mug's Flat as I could imagine trainee drivers being caught out by the only level section for miles.
Mugs flat was so called as it was the only reprieve between Helensburgh & Waterfall on the up. Not so much the drivers fault but owing to a lot of firemen not experienced with South Coast coal, by the time they got to that location with both driver & fireman trying to maintain steam & water, it was a good spot to stop for a few minutes to get a full head of steam before hitting the main drag to Waterfall.
From what I knew & heard of the spot, it was not just the inexperienced who would take the spot to blow up, Just in Case.