This is normal as the database is updating. If you click on developer menu above, and then choose Show logs, you'll see status information on what's going on.
Thanks! Also...
I had to go, so i closed the game, came back and reopened it and a database rebuild opened.
Also, its been 2 hours, and checking the logs, the rebuild hasn't gotten pastan asset called ; <NULL> : TrainzLockNativeFileSystem> resumable lock timeout: c:/users/(myname)/appdata/local/n3v games/trs19/build bi5jyyh71/packages/1068.tzarc ()
Not sure what this could be.
Possibly your AV software is preventing access?
1068 is a previewpass item. Maybe try uninstalling and reinstalling that package.
If you can't uninstall using Content Store, delete that file from /packages folder and rebuild the db.