The Terran Transit Marines (TTM) are responsible for defending the Terran Transit Authority's network of Transit Gates.

The Terran Transit Marines are stationed at transit gates. Answering to the corporation that runs a particular transit gate, their mandate is to maintain the safety of the corporation, its gate, its staff, and the population of the gate station, in that order. Because transit gates are so well-defended and damage to them could completely isolate a system, they rarely see action. Each company has its own unique, brightly striped colors in which they take extraordinary pride. A company’s beret is its badge of honor, earning a Marine both honor and jealousy from within the transit gate community.

Mobile Frame Zero: Rapid Attack Rulebook, pg 28