While "Borderlands Trainer" often refers to software used to enable cheats like infinite health or ammo, there isn't a single "good story" by that specific title. However, if you're looking for a narrative experience involving a "trainer" or ways to experience the Borderlands story more easily, here are the best options:
If your "story" involves a character you've spent years with, tools like WillowTree# serve as a different kind of "trainer." This save editor for the original Borderlands allows you to convert and edit savegame files across PC, PS3, and Xbox 360, effectively letting you write your own character's history by modifying levels and items. If you're interested, I can: borderlands trainer
The use of trainers is generally accepted in the gaming community when strictly applied to single-player campaigns. In Borderlands, players often use these tools to test weapon synergies, skip tedious leveling processes for alternate characters, or recover progress after a bug. However, using a trainer in online co-op (Borderlands Online) is strictly prohibited and highly frowned upon. Doing so can ruin the experience for teammates, destabilize the game server, and almost certainly result in a ban from the platform (such as Steam or Epic Games) or the game itself. While "Borderlands Trainer" often refers to software used