The restriction applies to the Eastbound at Kew Gardens only. A Refirbished D stock is 6 ton heavier than a non refirb, and this extra weight creates more sway of the car body when the train moves. The Platform at Kew Gdns must be tighter than others as this is the only one to have this problem. As a train sways less at lower speed, the restriction has been placed to minimise the risk.
I remember someone saying about how most new stock these days is much smaller than the older stocks (it was probably, but not necessarily, on this forum), and that was a reference to that. Particularly as the new stock is to be generic to all subsurface lines, and recently in railway terms where one-size fits all that size has generally been smaller than average.
The essential things in life are seen not with the eyes, but with the heart. --Antoine de St. Exupery
I doubt if it will be as much a 3ft though Chris. The old 'F' stock was 9' 7" wide and they had universal clearance on the combine. They were also IMHO the finest body design ever produced for ssl lines both internally in the seating arrangements and externally with door positioning. The only feature I would not retain is the oval windows.