It is named for Frances Appleton, Longfellow's wife.
Lukas insists on calling it the Longfellow bridge, so I guess he's more right than I knew. Maybe I can get him to call the Longfellow the Appleton.
When I was in college, the Longfellow was sometimes called the Salt and Pepper Shaker Bridge. But maybe Lukas has never seen shakers like that.
He sometimes calls it the Salt and Pepper Bridge in addition to Red Line Bridge. The former is presumably because I described it that way to him, the latter is totally his own thing.