ok, i am trying to make my setup identical to yours (as in
http://cosmos.infrae.com:8080/prefs_install_products_form).

You are using Archetypes 1.3.0-rc1 (CVS/UNRELEASED) but I've
installed Archetypes 1.3.0-rc2 and now I don't get the 
AT_GENERATE_METHOD import error. And I was able to install
PloneRailroad into my Plone site.

So I suspect that there have been some changes in the most
recent AT 1.3.1 which affect PloneRailroad.