This makes rake rails:freeze:edge work for your Rails 2 installation. find your rails gem then navigate to /lib/tasks/framework.rake ... locate a similar looking section and replace it with this

framework.rake

    desc 'Lock to latest Edge Rails, for a specific release use RELEASE=1.2.0'
    task :edge do
      require 'open-uri'
      require 'openssl'
      OpenSSL::SSL::VERIFY_PEER  = OpenSSL::SSL::VERIFY_NONE
      
      version = ENV["RELEASE"] || "edge"
      target  = "v#{version}"
      commits = "http://github.com/api/v1/yaml/rails/rails/commits/master"
      #url     = "http://dev.rubyonrails.org/archives/#{target}"
      url     = "https://github.com/rails/rails/zipball/#{target}"

      chdir 'vendor' do
        latest_revision = YAML.load(open(commits))["commits"].first["id"]

        puts "Downloading Rails from #{url}"
        File.open('rails.zip', 'wb') do |dst|
          open url  do |src|
            while chunk = src.read(4096)
              dst << chunk
            end
          end
        end

        puts 'Unpacking Rails'
        rm_rf 'rails'
        `unzip rails.zip`
        %w(rails.zip rails/Rakefile rails/cleanlogs.sh rails/pushgems.rb rails/release.rb).each do |goner|
          rm_f goner
        end

        puts "Frozen to git revision #{latest_revision}"
        File.open('rails/REVISION', 'w') do |revision|
          revision.puts latest_revision
        end
      end

      puts 'Updating current scripts, javascripts, and configuration settings'
      Rake::Task['rails:update'].invoke
    end