I suspect they have had the usual cost escalation once they started
confirming assumptions and looked at future train service patterns.
That's undoubtedly what has triggered the power and track related
changes on the NR tracks into Watford. There is a notorious lack of
asset knowledge in NR so it's no great shock that you get scope creep.
It is possible that there is a wish to run a more frequent service
into WJ and to Amersham. That may also have affected the power design
assumptions.

None of which is inconsistent with plain old "optimism bias" - something
the business case in 2009 played down[1] and the additions for which
some commentators (proponents) have simply omitted when citing costs.

[1] "As the project development of the rail link options has reached a
relatively mature stage, the optimism bias on capital costs applied
within the economic appraisal is 22%. Although this is lower than the
WebTAG recommended value of 44% at Programme Entry this is believed to
be entirely appropriate bearing in mind the extensive development work
undertaken previously on the proposals." Yeah, right.


--
Robin
reply to address is (meant to be) valid