On Wed, 11 Jul 2007 04:18:39 -0700, Mr Thant wrote:

Presumably only if you tell it not to count walks or cross-London
transfers as part of the shortest route.

Has it been established whether you should or not?

No, hence the options on CORE.

I emailed my Acton
Main Line to Clapham Junction scenario to NRE, and they said getting
the tube is shortest, but there's no instructions in the routeing
guide on which services should be considered when calculating the
shortest route.

The instructions in the RG do tell you to use the distances in the
National Rail Timetable. There are 3 possible interpretations:

-Only National Rail services should be considered.
-Cross-London transfers should be included, but only the distance on
National Rail counts (i.e. the Tube part of the journey is considered
to have a distance of 0).
-Cross-London transfers should be included, and you should use an
external source to find the length of the Tube part.

CORE gives you a choice between the first two of these.