My old Tarmac had the rear cable routed throught the top tube. It really isn't very difficult to get the cable routed. When you pull the old cable out if you can tie a peice of line to it you simply pull the new cable through with line. Still, I have a Cannondale Prophet and the rear derailleur cable enters the front of the swingarm and runs inside the full length to the rear. I just hunted and pecked with the brake housing until it came through. Still not hard to do. If nothing else, you could ask at your local shop or email Trek and ask them.

Mike