If it's on TS12, it may still be recoverable, but you will need to act fairly fast.
TS12 keeps a certain amount of days worth of backups in the <trainz install folder>/UserData/Backups folder and it's subfolders, which are in date order. Check in the folder on the date that it got deleted and check if it's in there. If it is, copy it's folder to the Desktop and use File > Import Content in Content Manager and point it to that folder.
If it's not in one of the backup folders, then you may be out of luck, as only the creator can send their assets unless otherwise stated in the config file.
Shane