Work around using any other version of Trainz 2.1 or above to increase the version number does work. Interestingly though pm2im can't read the pm file. Guessing that the forth coming overdue Murchison update, will just fix problems without a version update, which probably means downloading the whole thing again!
As an aside, I'm wondering if the other invisible items in Murchison are also using pm files, just a guess that maybe pm files don't like the now payware locked .textures. If I wasn't up to my neck in route building and getting my assets SP1'd, I'd go have a look.