Iron Rivers

A group of fairly primitive, nomadic tribals, the Iron Lines patrol and chart the railways of the pre-war world. They collect and often use handcarts to make their way across the wilderness. When they travel, they usually form handcart caravans for mutual safety. The Iron Lines resist the Daughters of Hecate and have remained largely immune to the plagues that haunt other tribes who go against Hecate’s wishes. However, the Vipers never pass up an opportunity to butcher any Iron Lines they discover. The Iron Lines decorate their bodies with paintings of parallel lines. Although they are nomadic, their main headquarters is Circle Junction, which is pretty literal in its naming - it's a circular collection or railways that intersect and go in a number of different directions, located east and south of Denver.

Appearances in games
The Iron Lines were intended to be in Van Buren, the cancelled Fallout 3 by Black Isle. Circle Junction is mentioned by Ulysses in Lonesome Road as a place the Courier has been.

Behind the scenes
While they were never referred to as "Iron Lines" during the actual development of the game, J.E. Sawyer later named them Iron Lines because of their body art and general association with the railroads, as he thought it was weird that all of the other tribes had names but they didn't.