About EdgeRails

Riding the Edge with EdgeRails

EdgeRails is a term which means you are running a local copy of a
developmental version of Rails (i.e., you’re on the bleeding edge).

source: http://wiki.rubyonrails.org/rails/pages/EdgeRails/

The easy way:
$ rake rails:freeze:edge
$ rake rails:unfreeze

As svn:externals (better?):
$ svn propedit svn:externals vendor
rails http://svn.rubyonrails.org/rails/trunk

As svn:externals, using a specific revision, which gives you more control -
you should figure out what revision is best for your app, of course.
$ svn propset svn:externals vendor
rails -r6881 http://svn.rubyonrails.org/rails/trunk

Using Piston (better, still):
$ piston import http://svn.rubyonrails.org/rails/trunk vendor/rails
or
$ piston import --revision=# http://svn.rubyonrails.org/rails/trunk
vendor/rails

By GEM:
$ gem install -s http://gems.rubyonrails.com rails -y