After exiting Surveyor and before quitting Trainz, I always create a .cdp file of any assets (route and/or sessions) that I have been working on. Those files I store in a backup folder on my main Trainz drive. I have another set of backup folders on an external drive, each folder is named according to the date it was created. After creating the .cdp files, I add a new folder using that days date to the backup drive and copy the latest .cdps to that folder. I then copy the new folder to a backup folder of another external drive.
During the development life of a project this gives me hundreds of backup copies and backups of backups, all named and stored in a way that makes it easy to go back to any date that I may need for an emergency recovery - and I have, on the odd occasion, had to restore a copy from days or even a week (or more) ago. Paranoia Pays!
Of course, once the project is finished and stable, then those backups can be deleted.
If you are using Windows 10 as your OS, then you can also take advantage of its File History feature to recover data that was saved months ago (depending on your storage capacity).