This is a common frustration with TS2010: you make changes to a session layer only but the game insists you must save the route and session layers. To be sure you can only alter the session layers it's a good idea to lock the route layers using the Layers tools tab in Surveyor and make sure a session layer is selected before you start editing.
If, after taking these precautions, TS2010 still insists you save a new route along with the edited session, go along with this, then close Surveyor and open CMP. Open the newly-created session for editing, and then open the 'config.txt' file in a text editor. Find the 'map-kuid' tag and change the kuid from that of the newly-created route to that of the Hawes Junction built-in route. Save the config changes then save the edited session. Now you can delete the newly-created route. Re-launch Trainz and you should find your newly-created session listed amongst the sessions for the Hawes Junction built-in route.