"Goodness!" cried Mary Louise. "I didn't know there was a subway in
the sea!"

"To be sure," answered the mermaid. "The track lies along the bottom
of the ocean. It's not a railroad train we're going to take, but a
water train that comes all the way from the Northern seas, sweeping
on like a river in the sea. Wait till we get down there. You'll
see how fast it goes."

Mary Louise was too astonished to speak.

"The Pullman cars," continued the mermaid, "are icebergs. They come
from the North every summer to take a trip South."

"Whew!" shivered Mary Louise. "I think we ar near one now, for I
feel quite cold."

Sure enough, she was right, for there close at hand was a great white
object.

"All aboard!" shouted a big polar bear. "Watch your step!"

The mermaid helped Mary Louise to slide on a projecting ledge, and
off they went.

"Now we can enjoy the scenery," laughed the mermaid, as she arranged
her tail in an artistic curve and brushed back her hair, which had
been swept over her eyes by the swift action of the water.

"The train never stops, you know, until it reaches its destination,