In message
,
Stephen Furley writes

They were, briefly, before they were removed. Marylebone had never in
it's history had more than four platforms before that time, and I
doubt if there was any reason to think that it wold need more in the
future. The redevelopment didn't reduce the capacity of the station,
it just eliminated the very wide and unneeded cab road. Why was it
built that wide in the first place?

I think it was just used to fill the space left when the original plans
for 10 platforms were scaled back to just 4, because of the escalating
cost of the GCR.
--
Paul Terry