This seems to be the oldest known source. From The Harvard Advocate (Cambridge, Mass.: Harvard College, Vol. 12, No. 3, Nov. 10, 1871), page 40:

[Note that this is given as a poem, not a song.]


IN THE TUNNEL.

Riding up from Bangor,
On the Pullman train,
From a six weeks' shooting
In the woods of Maine;
Quite extensive whiskers,
Beard, moustache as well,
Sat a "student feller,"
Tall and fine and swell.

Empty seat behind him,
No one at his side;
To a pleasant station
Now the train doth glide.
Enter aged couple,
Take the hinder seat;
Enter gentle maiden,
Beautiful, petite.

Blushingly she falters,
"Is this seat engaged?"
(See the aged couple
Properly enraged.)
Student, quite ecstatic,
Sees her ticket's "through;"
Thinks of the long tunnel,—
Knows what he will do.

So they sit and chatter,
While the cinders fly,
Till that "student feller"
Gets one in his eye;
And the gentle maiden
Quickly turns about,—
"May I, if you please, sir,
Try to get it out?"

Happy "student feller"
Feels a dainty touch;
Hears a gentle whisper,—
"Does it hurt you much?"
Fizz! ding, dong! a moment
In the tunnel quite,
And a glorious darkness
Black as Egypt's night.

Out into the daylight
Darts the Pullman train;
Student's beaver ruffled
Just the merest grain;
Maiden's hair is tumbled,
And there soon appeared
Cunning little ear-ring
Caught in student's beard.

S. O. L.