It seems that the releases are found under tags/releases
not trunk
. You could do svn list http://svn.boost.org/svn/boost/tags/release
to verify the release you are interested is there.
This is described at Development and Release Practices, where it indicates the URLs for final releases are in the form http://svn.boost.org/svn/boost/tags/releases/xxx, where xxx is a revision like 1_54_0.
The checkout command would be similar to:
$ svn co http://svn.boost.org/svn/boost/tags/release/Boost_1_55_0