gem install rails

May be it is a question of rem or ruby,but I need your help.
When I use gem install rails,it fails.But I use the same command,it
passed in another computer.
OK. I use
   hector@hector-laptop:~$ sudo gem install rails --no-rdoc --no-ri
The result:
   source gems.rubyonrails.org not present in cache

But I try to use wget to get gems.rubyonrails.org. Anything is okay.
Here is my environment:
gem environment
RubyGems Environment:
  - RUBYGEMS VERSION: 1.1.0 (1.1.0)
  - RUBY VERSION: 1.8.6 (2007-09-24 patchlevel 111) [i486-linux]
  - INSTALLATION DIRECTORY: /usr/lib/ruby/gems/1.8
  - RUBY EXECUTABLE: /usr/bin/ruby1.8
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86-linux
  - GEM PATHS:
     - /usr/lib/ruby/gems/1.8
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://gems.rubyforge.org

Sounds like you have different sources on the two machines. There was
talk recently about where best to source for rails, but I don't
remember quite what the answer was.
Anyway, you want to run this on both machines:
$ gem sources

Then you can add missing sources (like the one gem is complaining
about above, gems.rubyonrails.org) with:
$ gem source <gem source location>

-Dale