Yes, nasty. That plus the panic that often follows such an event (been there, done that!).
For future reference (should it ever happen again

) - the automatic backup .tzarc files are always in the
Date-of-last-use (yy-mm-dd format) folder in the
backups folder under your Trainz local user data folder, which you can find in the Trainz Settings window Install tab. The recovery method is to drag the required tzarc file (which will be named using the <kuid> of the route) into the editing folder, which is also under your Trainz local user data folder alongside the
backups folder, and then perform a DBR. Full details (with pictures) can be found on the Trainz Wiki at
https://online.ts2009.com/mediaWiki/index.php/How_to_Manage_Backups