The assumption visible in the
RUS▸ and Route Study was for electrification and 3-car trains, with no significant work. Officially Platform 1 is 67 m long, so it could take 3x20 m, but not quite 3x23 m. As to where a few 3-car
EMUs▸ might be found, presumably that was someone else's problem.
The 2019 service is assumed to be 1 tph with a second in the peaks, but no longer running through to Paddington. To run 2 tph through to Marlow, passing at Bourne End, you would need Platform 2 to be connected to Marlow far enough up (60 m) to be used too. Is suspect that's feasible for its radius, but of course it would have to be built and paid for.
In any case, what is discussed is the interchange working as at present. Of course that permits the Maidenhead-Bourne End train to be longer, up to 5-car with the 100 m available (and at Furze Platt and Cookham too).
The proposed all-day service to be introduced before 2043 is 2 tph, but by that date more capacity is needed as far as Bourne End. The conclusion here reads:
By 2043 peak demand on the Marlow branch will exceed capacity
of the 2019 baseline service frequency between Bourne End and
Maidenhead. Train lengthening beyond 3-cars would address
on-train crowding but is likely to require significant infrastructure
work at Bourne End to increase the length of Platform 1. Increasing
the frequency of the train service to 2tph to meet the connectivity
Conditional Output would also deliver the required peak capacity
whilst removing the need for infrastructure intervention, should an
interchange at Bourne End be made. The longer-term strategy is
therefore to deliver the connectivity Conditional Output on the
Marlow branch.
That's not the clearest, but I think it says 2 tph can be committed to since if they can't stretch platform 1 they can run the peak-hour service all day. How so? The peak capacity can only be improved by more or longer trains on the peak service, which is already 2 tph. So perhaps they are relying on longer trains to Bourne End already.
What isn't mentioned is that each half could quite easily run 3 tph, and even 4 tph in the peaks at a push. And, of course, if you accept Marlow is served by a
DMU▸ shuttle, you can still electrify to Bourne End (Platform 2 anyway).
I'm sure I've seen a suggestion (possibly from Marlowites) that a new turnout and chord from Platfrom 2 to Marlow could just about allow 80 m of usable platform. It would be a question of whether the smallest radius you dare use would just fit, or whether you might have to beg a sliver of land. But I can't see that attracting any enthusiasm, at least in the short term (which could be over ten years if you are
NR» ), as it would only add to the amount of electrification on the list.
PS: Of course to pass two 4-car trains at Bourne End you'd still need Platform 1 extended to that length anyway. And then there's Marlow, though there's plenty of space to lengthen there. So what about a nice little Swiss-style tram?