I have wondred about the rolling stock for crossrail services but one
question i have is that couldn't some 2/3 carriage EMU'S be built so
that peak hour trains can be split/attached so that they can serve
some of the thames valley branches instead of having a shuttle on
those lines?

One such route i am thinking about is the Maidenhead to Marlow branch,
doesnt this have through services to London during peak hours?