Loop Mod is an addition to Black Mesa's On a Rail chapter, adding a couple areas to the very first map of the chapter. The mod reinstates the looped gameplay that was present in the original Half Life but was left out of Black Mesa, in part because the original design was deemed confusing. This issue has (hopefully) been done away with in the mod. It also reinstates three iconic areas from the original as well. The mod also fixes some design errors that were present in the original map. Comes with On a Rail Uncut compatibility.
As you already know, the Loop Mod adds a loop into the first map of the On a Rail chapter in Black Mesa that was not in the original version of that map.
If you want the first map in the On a Rail chapter of BM to be more true to Half-Life 1, download this mod. It is a great addition the game.