Hi, Acme. I learned it as "a mule in the mine," but it looks like the printed sources prefer "the mule in the mines." Here's a school songbook version:

MY SWEETHEART'S THE MULE IN THE MINES
Pennsylvania Coal Miner's Song

My sweetheart's the mule in the mines,
I drive her without any lines;
On the bumper I stand,
With my whip in my hand.
My sweetheart's the mule in the mines.


Source: Music in Our Country, Silver Burdett Music For Living Series, book 5, page 63

notes: Before electricity was installed underground, mules were used to pull the flat-cars that transport mined coal from the digging area to the shaft up which it Is lifted to the surface. Boys in their early teens drove these mules. Early in the morning they got their mules from the stables, fed them, then hitched them to "trips" of empty cars. With a crack of the whip and a shout, each mule train started off down a passage so dark only the mule could see what lay ahead. Thus, the driver had no reins; he had to govern his mule with the tone of his voice. The boys had to work underground for ten hours. They sang this and other songs to brighten the long day.


I wonder if the song was "sanitized" for school use.