How could the contextual filter "Content: Nid" return the TNID instead of NID?
This PHP code should work:  

$node = node_load(arg(1));
return (isset($node-&gt;tnid) ? $node-&gt;tnid : $node-&gt;nid); 


The better way (without evil eval()) is to provide own plugun of type 'argument default' which will do work. Look at Content ID from URL argument default plugin as an example.