The milestone import already has it's own wiki section within the ProRail import wiki.
Which is here: http://wiki.openstreetmap.org/wiki/ProRail_import
Your thoughts on the milestone import and milestone locations are highly appreciated. Thank you in advance.
Well, you could start with one or 2 railways that are single track anyway, so you could iron out problems that could come up at greater style imports. Since you need to hand-check for already existing data and merge those nodes back into the tracks this will need a lot of steps anyway I guess.
Don't forget to run this on a dedicated account and document everything on the wiki (source of the data, license, whatever). I personally would entirely leave out the "do not touch" or "source" comment and just attach that to the changeset comment.
Those locations should also have a dot as decimal separator, you had a komma in your Excel sheet. It's already with a dot in the wiki.
Eike