David Walters' post says the original plan was to use the Great
Northern station. (Mind you, plans often change. The original
plan for the Victoria Line was that it would start in Croydon and
end up in Chingford)

However, the line from Highgate Station around the north side of
Highgate Wood was in a deep, deep cutting, which is probably
why the fence is maintained in such good order. Climbing from
the Underground Station to a convenient place in that cutting
would have been manageable.