Locomotive is a simple solution for getting up and running for Ruby on Rails development on your Mac. But like Instant Rails, a similar but not exactly the same project for Windows, eventually it got tiresome to maintain and the person who started the project has quit. Unlike Instant Rails, no one has yet stepped forward to adopt Locomotive.
I really hope somebody does continue the project for a while. The Ruby on Rails site suggests a long series of steps you can follow to compile Ruby and install Rails on your Mac. It works but it really has nothing to do with developing a website. It’s just one more impediment to keep somebody with an idea from getting started developing that idea. Apple saw that and includes both Ruby and Rails with OS X Leopard, the latest version of the operating system that runs on Macs, but for the many users still on the last version we still need an easy quick start for a while longer.