Mainly, xtrn doesn't depend on the revision control system. This means that you can pull in code from heterogeneous systems with a single command.
Just create an Externals file in the root of your project, require the xtrn gem and run xtrn (again, in the root of your project).
The format of the Externals file is as follows, note that the username and password will not be cached:
---
- url: svn://svnhost/dir/to/external/codebase
path: checkout/to/here
type: svn
username: optional_username
password: optional_password
- url: svn://svnhost2/another/thing
path: this/one/goes/here
type: svn