wheels, the locomotive's weight is distributed more evenly along the rails. Some of the six-wheel trucks
use four wheels as driving wheels and two as idling wheels, while in other six-wheel trucks all six are
driving wheels. Idling wheels merely help to equalize the weight distribution of the locomotive.
The locomotive engine supports various accessories such as the horn and windshield wiper.
These are actuated by either mechanical or electrical means to parts of the engine.
a. Bell. The signal bell is stationary with a movable clapper operated by an air valve located at
the engineman's station. The bell is usually located under the floor behind the pilot or switchman's
footboards on the right side of the front end of the locomotive.
b. Horn. The horn, operated by air valves that allow air to be forced through the sounding
device, is mounted on top of the cab and controlled by two pull cords above the control stand. One cord
gives a soft tone and the other a full tone. The horn shutoff cock is in the air line above the floor in front
of the controller. This air line supplies the horn valves with air.
c. Speed recorder. The speed recorder, similar to a speedometer, can be operated hydraulically,
mechanically, or electrically.
d. Windshield wipers. Windshield wipers are operated by air from the compressed air system.
They are controlled by valves over the cab windows and operate independently of each other.
Windshield wipers should never be run on a dry window as dirt on the window will scratch the glass.
e. Sanding system. A sanding system delivers sand between the wheel and the rail. It has three
parts: the sand trap, the control valve, and the operating device. Sand is kept in a box and flows through
the sand trap and pipe to the rail. In some locomotives, the sanding system can be operated by the
automatic brake valve handle. In others, it is operated by hand.
f. Temperature controls. On larger locomotives, water temperature in the engine cooling
system is regulated automatically. When water temperature changes, a thermostat operates switches
which activate fan motor contactors. As the temperature increases, a medium-speed relay closes; further
increase closes a full-speed