I tried using the tarball releases of Railroad and
PloneRailroad (instead of the svn checkouts) and also
updated to the latest Archetypes 1.3.0-final, but I'm still
getting the error.

I issued the "grep -r AT_GENERATE_METHOD *" command in the
Products dir and discovered that AT_GENERATE_METHOD is
declared in ATRepresentations/utils.py, but
ATRepresentations is not mentioned as a dependency.

It's also imported in Archetypes/public.py:
from Products.Archetypes.ClassGen import AT_GENERATE_METHOD: