THC
Member
The fundamental point is that I don't think they see any prospect of there being enough traffic north from Amersham to justify a through service to MK in the first place.
Interestingly enough, this is not quite true. In the labyrinthine approvals process for the Metropolitan line extension (née Croxley Rail Link), a letter from the DfT to the GLA of 30 March this year (see p18) states that, as a condition of DfT funding:
Department for Transport said:Transport for London will assess in good faith and agree with the Department... the viability of the infrastructure accepting the operation of national rail DMUs (Class 165, 168, 170 and 172 up to six-cars in length), recognising DfT and stakeholders have a longer term aspirations for a service of 2 train per hour from Watford Junction to Rickmansworth, Aylesbury (and beyond).
So, no further information on progress with this (which itself was subject to consultation with TfL) or timescales, but it is the first confirmation of official interest in i) services from Watford Junction northwards over the Croxley North Curve, and ii) National Rail services from Watford Junction to Aylesbury and potentially beyond.
THC