a. Third rails are generally used to carry direct current at comparatively low voltages, about 600 volts,
as well as high current. They are usually well shielded to prevent personnel from coming in contact with them.
Third rails must be de-energized before any major track repair work is done near them. Minor work, such as
raising a low joint, may be done with the power on if experienced crewmen do the job. Often, a set of electric
lights is connected between the third rail and the running rail. While these lights are out, the power may be
assumed to be disconnected.
b. Overhead trolley wires, supported by a catenary system,* may carry current similar to that carried by
third rails, or they may carry high-voltage alternating current, approximately 11,000 volts. Such high voltage is
extremely dangerous because it may jump through space--arc--as much as several feet. In addition, the mere de-
energizing of such lines does not make them harmless, because they retain a residual charge. This makes it
mandatory that they be grounded after being disconnected from the power source. Track maintenance men must
not come within 3 feet of energized or ungrounded de-energized high-voltage overhead lines. Crane booms or
other maintenance-of-way equipment must not be brought within 8 feet of such wires.
Tunnels, bridges, interlocking plants, railroad junctions, and electrified railways create special
maintenance problems. The limited space in tunnels makes track maintenance jobs difficult; everything possible
should be done to reduce the amount of repair work needed in them. Welded rails eliminate joints and using
concrete ties avoids the frequent replacement of wooden ones. Whenever track maintenance in a tunnel becomes
necessary, out-of-face instead of the spot method should be used, allowing the entire job to be completed at one
time. However, remember that out-of-face track maintenance can change the height clearance of the tunnel.
Maintenance on bridges is always difficult, particularly on open-floor bridges where there is no ballast.
To simplify the vertical and horizontal movement of the track, each tie must be custom made for each individual
replacement. Bridge and building personnel
*A catenary system is a method of supporting a trolley wire horizontally by suspending it by messenger wires
from a catenary--a curve formed by a cord or cable hanging freely from two fixed points or supports.