TRN file contains transaction logs for Microsoft SQL Server relational database that record all completed and pending transactions since last checkpoint. TRN file is a type of backup data storage that along with BAK file, which contains database data, can be used to recover database in the event of a failure or loss.

Microsoft SQL Server, a relational database management system software, utilizes chain of TRN files to recover database by performing a rollback to previous restore point set at particular point in time. Log files can be stored using file formats other than TRN.