I have a feeling that, for at least the South-NW line, they will plan ahead for a future expansion to 5 car trains, at least for the stations where doing so might be a problem in the future. There is no sense doing a major rebuild of a station only to have to do another major rebuild 10-15 years down the road. The stations that will need major work to get to 4 car length will be rebuilt so that the extension to 5 cars is easy.
I don't think that the NE-West line will ever require 5 car trains - if it does, by that time, the whole line might need to be rebuilt and put underground anyway.
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