In Churchill’s history of ww2 he says there were 6 ISD‘s under construction in 1940. I have found Wapley for Bristol Docks now housing estate, Simonswood for Liverpool,, St Mellon for Cardiff,and Deanside for Glasgow all now industrial estates but where are the other two. Wrangaton for Plymouth is one possibility but it is very short of railway sidings. The known ones are all similar 4x long double apex sorting sheds all rail served plus lots of sorting and reception sidings.