Although the first printed version of the song appeared in Carl Sandburg's The American Songbag in 1927, the song is believed to have been written during the late 1800s. The song was based on an old Negro spiritual titled "When the Chariot Comes", which is sung to the same melody. During the 19th century it spread through Appalachia where the lyrics were changed into their current form. The song was later sung by railroad work gangs in the Midwestern United States in the 1890s. The song's style is reminiscent of the call and response structure of many folk songs of the time, where one person would shout the first line and others repeat.

It refers to the Second Coming and subsequent Rapture. The she refers to thechariot the returning Christ is imagined as driving.

We know it as “She’ll be Coming Around the Mountain”.

http://en.wikipedia.org/wiki/She'll_Be_Coming_'Round_the_Mountain

LYRICS:  When the Chariot Comes

(Music - She'll be Coming Round the Mountain)

O, who will drive the chariot When she comes?                                                                           O, who will drive the chariot When she comes?                                                                           O, who will drive the chariot, O, who will drive the chariot,                                                            O, who will drive the chariot When she comes?

King Jesus, he'll be driver when she comes, When she comes. . . . 

She'll be loaded with bright Angels When she comes . . . .

She will neither rock nor totter, When she comes . . . .

She will run so level and steady, When she comes . . . .

She will take us to the portals, When she comes . . . .