steel ribbons for a dozen miles, from the coal beds to water and
railroad transportation. Put a half dozen loaded cars on the track,
and with one man at the brake, lest gravitation should prove too
willing a helper, away they go, through the springtime freshness or the
autumn glory, spinning and singing down to the point of universal
distribution.

[Illustration: Incline at Mauch Chunk.]

On one occasion the brake for some reason would not work. The cars
just flew like an arrow. The man's hair stood up from fright and the
wind. Coming to a curve the cars kept straight on, ran down a bank,
dashed right into the end of a house and spilled their whole load in
the cellar. Probably no man ever laid in a winter's supply of coal so
quickly or so undesirably.

But how do we get the cars back? It is pleasant sliding down hill on a
rail, but who pulls the sled back? Gravitation. It is just as willing
to work both ways as one way.

Think of a great letter X a dozen miles long.

Lay it down on the side against three or four rough hills. Bend the X
till it will fit the curves and precipices of these hills. That is the
double track. Now when loaded cars have come down one bar of the X by
gravity, draw them up by a sharp incline to the upper end of the other
bar, and away they go by gravity to the other end. Draw them up one
more incline, and they are ready to take a new load and buzz down to
the bottom again.