Who put the “L” in the Loop? If you’re talking about the elevated tracks that circle the area, then Charles Tyson Yerkes gets much of the credit. He championed building elevated tracks to bring in trains that had previously stopped outside of the area.
It’s important to note, however, that there was a Loop before there was an “L” in it; earlier, steam-powered cable cars circulated downtown, giving the area its name prior to the coming of the elevated tracks and trains.