Easy one!
Thw route from Maidenhead to Bourne End is passed for DOO▸ (Driver-Only Operation (that is, trains which operate without carrying a guard))(P) operation and the Bourne End to Marlow route is not so therfore when they are short of a guard they can run Marlow services DOO(P) to Bourne End.
They are two reasons for this - the ground frame has to be operated to get to Marlow and this needs a Conductor (driver could do it but ASLEF» (Associated Society of Locomotive Engineers and Firemen - about) would probably argue and needs more time as driver would then have to set cab up). The other reason is that DOO(P) regulations only allow you to operate away from a track circuited line for a maximum of 20 minutes and the journey from Maidenhead to Marlow exceeds that.
The peak hour shuttles to Bourne End are DOO(P) and the day time branch is DOO(P) operated between Maidenhead and Bourne End allowing the Conductor to focus on tickets!
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