I had wondered
Aren't London and South East fares a lot higher than in the rest of the country?
As I said in my original reply, on such a short journey 313s and their ilk would be perfectly acceptable, and from using that line, it feels like a rural branch line rather than a major commuter route anyway as it chugs slowly along behind allotments before crossing that lovely plain next to the Thames
London & SE fares tend to be higher than other areas:
a) because the majority of the passengers have higher overall costs and incomes
and
b) because the services tend to be more frequent and faster (as a consequence of a much higher volume of travellers)
Because of this much higher-level of service need, the rolling stock and lineside systems need to have more resilience and greater reliability than can be tolerated elsewhere in the UK. Failure to provide such resilience produces a breakdown in services wherever it happens. In the southeast, when trains carrying over 1000 passengers, running on less than 5 minute headways breakdown on infrastructure that doesn't have bypass routes, disruption can affect 10s of thousands of passengers with very little chance of recovery within the peak period.
Now if we focus on the few minor branch lines that feed into the main routes into London, problems on most of them can at least be contained and not cripple the lines that they feed. The Marlow branch is just one of those, i.e. if a train fails anywhere on the branch, including Maidenhead platform 5, it will not bring down either Crossrail or GWML services. As a consequence, any reasonably serviceable stock can run the branch timetable, certainly as an interim measure.