That doesn't really answer the question though- the situation at Bourne End places a constraint on train length, so any fleet plan that doesn't consider this as part of the picture will need an exception to deal with that constraint.
If a plan specifies 4-car units for all the Thames Valley branches, Marlow would be left unserved unless you have a microfleet of 2-car units to cover that part of the network. Either that or you have to spend a large chunk of money to allow those 4-car units to operate there.
Not sure if there is enough units, parts etc... But I think you are more likely to have something like a class 230 along the Marlow/Bourne End route.
The only branch route where you could run a four car train is the route to Henley-On-Thames. This could potentially be a battery operated train, whether it is class 387 or 379.
Now, if finance was available an ideal option would be a two/three car version of class 555 or 777 for both Marlow/Bourne End, Windsor and Greenford routes and possibly Henley-On-Thames or use a similar Metro platform from another manufacturer such as Alstom Metropolis or Movia (London Underground S Stock) or Siemens Inspiro. Another option would be to convert the LU 1992 stock, that within the next few years is going to be withdrawn, to be replaced with Siemens Inspiro based trains. But that is probably a discussion for another thread.
