Since locomotive is a developer tool, I decided to install it in /Developer/Applications/Locomotive2. I also manually added the rmagick bundle's framework/bin directory to my path before /usr/bin so that it would pick up the bundle's version of Ruby before Apple's built-in one so that I could use it for other tasks too.
However, when I then try to run `irb` it reckons that it can't require 'irb'. Checking out the load path, with:
ruby -e 'puts $:'
reveals that the load paths are all prefixed with /Applications/Locomotive2/...
It's not that big a deal -- I can cope with it being installed in /Applications and I'm sure most of the world does have it that way. However, it might be a good idea to update the documentation to reflect this fact.