Versions of Rails


I am using fedora 8 with Rails version 2.0.2. But i need to change to
version 0.12.1. I have loaded 0.12.1 with

gem install rails --version 0.12.1

It went fine and now i have 2.0.2 and 0.12.1 verisons, but when i try
to add this linse to config/envrionment.rb

# Require Rails libraries.
require 'rubygems'
require_gem 'activesupport', '= 1.0.4'
require_gem 'activerecord', '= 1.10.1'
require_gem 'actionpack', '= 1.8.1'
require_gem 'actionmailer', '= 0.9.1'
require_gem 'actionwebservice', '= 0.7.1'
require_gem 'rails', '= 0.12.1'

these lines are after line
require File.join(File.dirname(__FILE__), 'boot')

and rebooting after those changes, rails will start but will give
error : Rails application failed to start properly

Can somebody help me to use version 0.12.1 in my application

Sorry about the wrong group... I did new post to the talk...