That's just one 720 by 720m tile. You can add more terrain in Surveyor edit mode by simply clicking near the edge of the block after selecting the
Add/Update ground 'X' tool of the Topology Add section tool tab (F1 key).
That way you can have plenty of space to grow your railroad!
For more information, check out this article in the Trainz Wiki:
https://online.ts2009.com/mediaWiki/index.php/Route_and_Session_Basics