Doesn't STA Travel and the like focus on international routes where true "wholesale" fares are available? Seems like using them on a domestic U.S. route would be less effective.

I used STA back in the early 90's to buy tickets back and forth between Wales and Kansas City. Every time I checked with the airlines, STA would find something significantly less expensive for me. Plus they could obviously sell all of the necessary train segments at the same time...