I have always thought that the platforms at St. James's Park seemed unusually long. The eastbound platform may have been extended so that 8-car trains could stand clear of the trailing crossover formerly at the west end of the station. However, the westbound platform extended across the site of the crossover, well past the current stopping mark. That platform would appear to be much longer than an 8-car train could ever need. Was this connected to the station being sited under the former HQ? Any explanation would be appreciated.
From the old maps linked in the S.Kensington thread link, it looks like pre-55 Broadway the platforms ended with stairs to an entry by the Queen Anne's Gate building. Also some extra track and pointwork at the west end of the station.
Expanding the older map, it looks like to the west of the station, in addition to the trailing crossover which lasted until 1967, there appears to be a trailing siding from the westbound line which enters a bay platform before disappearing under the overall station roof. What would this have been provided for?
LT Museum has many photos of the original station entrance opposite current road known as Queen Anne's Gate which stairs reached from the east end of the original platforms. e.g. link Current entrance is some 200 feet to the east and platforms now extend to east of those access stairs. Original drawing shows platforms starting to west of station entrance here and overall roof seen here