Dreamhost's default setup places all domains and subdomains in the top-level of your home directory, but this can be changed when you add the domains or at any time after. These folders are the DocumentRoots for your domains.
I don't use Subversion or Trac on DH servers, but I use both at home. Trac does not support multiple projects, so you'll need to set up Trac directories for each one separately, which also means separate Subversion repositories. I use the a directory structure like this, giving my Trac instances the same name as their corresponding svn repositories:
This could easily be adapted to work here by putting the svn and trac directories into your home directory. This only requires one Trac installation; you can use .htaccess to set up your Trac environments on a per-site/directory basis.
Using multiple Subversion repositories also gives you an easy way to grant access to others on a per-project basis. I group related projects together and put anything that may need unusual permissions into its own repository.
DocumentRoots for Rails sites can be set up the same way as any other site, with one small change. Create the project in its domain folder like your other sites, then modify the root directory in the panel to include the 'public' subdirectory (ie, ~/example.com/public).
It's not everything you've asked for, but I hope this helps get you started.
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