By the title and the rhythm of the song, I'd say the "she" must originally have referred to a train - as Guest said a few years back.It would have been sung as a welcome to Mother Jones later.   The 600 horsepower bit is a bit of a stretch though- nobody would have "got" that sort of information overload in the song . A hearse coming round a mountain being pulled by six white horses doesn't make any sort of sense.