There was, for a couple of years 1878-1880, a Midland operated Super Outer Circle service, from St Pancras via the Dudding Hill Loop to Earl's Court which used it (an earlier variant had taken Midland trains to Richmond); and it also provided access to a second Midland coal depot on the MDR, at High St Kensington.As far as I know, the line from South Acton to Turnham Green was just for the coal trains to and from the Midland coal yard adjacent to Lillie Bridge LT yard. There used to be a line off the down District at West Kensington, as can be seen on this map.
The history of the railways of Kew and Gunnersbury was detailed (by J C Gillham) in the Railway Magazine of August and September 1956.