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.
