The knife used by Hallam (Benicio Del Toro) throughout most of the movie is a Beck Wilderness Survival Knife, designed by Tom Brown Jr. (a tracker/survival expert who was a technical advisor on this feature) and constructed by David Beck.
The Metro Area Express (MAX) light rail train used on the Hawthorne Bridge scene was actually two articulated buses joined together at the rear ends, with the frontal portion of the "train" to look like the actual older Bombadier MAX trains.
In the scene where Agent Hewitt hands Van Zandt a letter demanding Hallams release. The letter is signed by Attorney General Scott A. Anderson. Scott M. Anderson was the assistant property master for the film.
When Hallam tells Bonham the men he killed weren't hunters, "they were sweepers", he means he thinks they were sent to kill ("sweep") him. William Friedkin says on the DVD audio commentary that it's left ambiguous whether that is true.