Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Trans-tĭbĕrīnus, a, um, adj., that is beyond the Tiber, Transtiberine: ambulator, Mart. 1, 42, 3.
Subst.: Transtĭ-bĕrīni, ōrum, m., those dwelling beyond the Tiber, Cic. Att. 12, 23, 3.