All traction engines to be practical must of a necessity, be reversible.
To accomplish this, the link with the double eccentric is the one most
generally used, although various other devices are used with more or
less success. As they all accomplish the same purpose it is not
necessary for us to discuss the merits or demerits of either.

The main object is to enable the operator to run his engine either
backward or forward at will, but the link is also a great cause of
economy, as it enables the engineer to use the steam more or less
expansively, as he may use more or less power, and, especially is this
true, while the engine is on the road, as the power required may vary in
going a short distance, anywhere from nothing in going down hill, to the
full power of your engine in going up.

By using steam expansively, we mean the cutting off of the steam from
the cylinder, when the piston has traveled a certain part of its stroke.
The earlier in the stroke this is accomplished the more benefit you get
of the expansive force of the steam.

The reverse on traction engines is usually arranged to cut off at I/4,
I/2 or 3/4. To illustrate what is meant by "cutting off" at I/4, I/2 or
3/4, we will suppose the engine has a I2 inch stroke. The piston begins
its stroke at the end of cylinder, and is driven by live steam through
an open port, 3 inches or one quarter of the stroke, when the port is
closed by the valve shutting the steam from the cylinder, and the piston
is driven the remaining 9 inches of its stroke by the expansive force of
the steam. By cutting off at I/2 we mean that the piston is driven half
its stroke or 6 inches by live steam, and by the expansion of the steam
the remaining 6 inches; by 3/4 we mean that live steam is used 9 inches
before cutting off, and expansively the remaining 3 inches of stroke.