My rails project has this line in /config/environment.rb
# Specifies gem version of Rails to use when vendor/rails is not present
RAILS_GEM_VERSION = '2.3.2' unless defined? RAILS_GEM_VERSION
No, there isn't.
You application needs to use a specific Rails version mostly because different tiny releases might require additional steps to upgrade the framework such as changes to boot.rb.
$ rake rails:update