Broadly speaking, wayside distribution systems can be subdivided into the overhead contact
system and supplemental cabling systems to connect the
catenary to the traction power substations. The latter is the topic of this article.
Each TPSS is typically linked to the trackway by underground conduits. One set of conduits runs
to and up one or more catenary system poles to carry the cables that provide power to the
catenary. The conduit risers can be located either on the outside surfaces of the OCS poles or
within the poles, either of which can require an appreciable foundation at trackside. Once the
power supply is terminated to the catenary, the positive side of the traction power supply
distribution usually remains on aerial structures and does not interface further with the track.
Another set of conduits and cables runs to the track and provides the negative return path for
traction power back to the TPSS. The design of the trackbed needs to accommodate these
traction power supply system conduits. Adequate space is required beneath the track for the
conduit systems (including terminations), conduit risers, and manholes. The track itself must
accommodate connections of the negative return cables.
If the overhead contact wire system is a single filament trolley wire, it is usually necessary to have
supplemental feeder cables as well so that the overall traction power distribution system has
sufficient electrical capacity to provide current without unacceptably large voltage drops. In urban
areas, these feeder cables are, for aesthetic reasons, most often routed through underground
duct banks that run parallel to the tracks. The feeder cable must be periodically connected to the
trolley wire, usually at every third to fifth OCS pole. At each such location, a manhole will be
located along the trunk duct bank and a branch conduit will run over to the OCS poles. The
overall design of the trackway must accommodate these ducts and manholes.
A messenger/feeder can be placed above the trolley to obviate the need for parallel feeders. The
messenger-to-trolley vertical dimension (construction depth) can be made small (6” to 12”) to
reduce visual impact, with no effect on track design.
Less often, supplemental feeder cables are carried on the OCS poles rather than being routed in
underground conduits. Many legacy streetcar lines used this configuration. This substantially
reduces the impacts on the trackway design, but the overhead cables negate some of the visual
aesthetic benefits of a single filament trolley wire system as it effectively just moves the
trolley wire running surface must be continuous, without any gaps or overlaps. In addition, trolley
hardware for pole operation is not normally suited for pantograph operation.
A simple catenary system uses a messenger wire to support the horizontal trolley wire. Both
conductors are used to transmit power from the substation to the vehicle. In a simple catenary
system, the system height at the supporting poles—the distance between the contact or trolley
wire and the messenger—is approximately 4 feet [1.2 meters]. In tangent track, this allows spans
between poles of up to 240 feet [73 meters].
The low-profile catenary system is similar to the simple catenary design, except that the system
height at the support is reduced to approximately 18 inches [457 millimeters] and sometimes less.
This style is often applied in aesthetically sensitive areas where a lower profile and simple singlewire
cross spans are more desirable. The trade-off, however, is that the span length between
supporting poles is reduced to approximately 150 feet [46 meters].
Single filament trolley wire systems, which were traditionally used on legacy streetcar lines, are
considered by many persons, and particularly by lay audiences, to be visually less obtrusive in
the urban environment. It provides power through a single trolley wire that must be supported at
least every 100 feet [30 meters]. The span length is limited by the sag of the unsupported trolley
wire, which in high temperatures could otherwise fall below the minimum elevation required by
the National Electrical Safety Code. It is also limited by the structural capacity of the supporting
hardware to carry the weight of the entire length of a span between supports. Single filament
trolley wire also usually requires the wire to be supplemented electrically by parallel feeders that
must be frequently connected to the trolley wire to maintain sufficient voltage for LRV operation.
These feeders may either run underground through a series of ducts and manholes, which are
expensive, or be hung from poles. The latter position can arguably be just as visually intrusive as
the messenger wire in a catenary system and merely relocated to a different point in the
observer’s line of sight. The overhead wire design to accommodate trolley pole operation
requires more support and registration points and can therefore have nearly twice the number of
poles than the equivalent simple catenary system.
