Here are some notes from SI539 – when you upgrade to Leopard – pretty much you no longer need Locomotive as it has plenty of Ruby and Rails built-in.
$ rails –version
Rails 1.2.3
$ ruby –version
ruby 1.8.6 (2007-06-07 patchlevel 36) [universal-darwin9.0]
Here are some notes as to how to use it instead of Locomotive – actually now DOS and Mac are even more similar.