Ben Wear from the Austin American Statesman wrote an article on his blog today on the proposal for an Austin starter line by Roma design. In the article he muffed the overhead wire, not my blog, but the technology details.
Light rail, as opposed to the commuter rail opening late this year or early next year, is generally powered by electricity and has a system or overhead wires that connect to devices on the top of the cars called “catenaries.”Sigh. The devices on the top of the vehicles can be two things, Pantograph or a Trolley Pole, just like my nom de plume. Interesting history point, the pantograph was actually invented by none other than the east bay Key System, now AC Transit, which had it's 50th tear down anniversary last week. The catenary is actually the support wire for the electric wire. From wikipedia:
To achieve good high speed current collection, it is necessary to keep the contact wire geometry within defined limits throughout the length of the overhead line. It is usually achieved by supporting the contact wire from above by means of a second wire, known variously as the messenger wire (US & Europe) or catenary (UK & Canada). This wire is allowed to follow the natural path of a wire strung between two points, which is known as a catenary curve, thus the use of catenary to describe this wire or sometimes the whole system. This wire is attached to the contact wire at regular intervals by vertical wires known as droppers or drop wires. In this way the contact wire is effectively supported at numerous points.Sometimes we also call simple equipment a trolley wire which is much more aesthetically pleasing and should be used in downtown settings to minimize the visual pollution. I know this is a bit picky, but if you're going to continuously pick on the local transit agency on details, at least get yours straight.
The messenger wire is supported regularly at structures, either by means of a pulley, link, or clamp. The whole system is then subjected to a mechanical tension. The messenger wire is usually pulled slightly to the left and right by successive supports, so that the contact wire slides from side to side(stagger) on the pantograph as the vehicle moves along (if it did not then it would tend to wear a groove in the pantograph's carbon insert). Such a system, with a single supporting wire, is known as simple equipment.